3513 lines
135 KiB
Plaintext
3513 lines
135 KiB
Plaintext
@c Copyright (C) 1988,89,92,93,94,96 Free Software Foundation, Inc.
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@c This is part of the GCC manual.
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@c For copying conditions, see the file gcc.texi.
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@node C Extensions
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@chapter Extensions to the C Language Family
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@cindex extensions, C language
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@cindex C language extensions
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GNU C provides several language features not found in ANSI standard C.
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(The @samp{-pedantic} option directs GNU CC to print a warning message if
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any of these features is used.) To test for the availability of these
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features in conditional compilation, check for a predefined macro
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@code{__GNUC__}, which is always defined under GNU CC.
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These extensions are available in C and Objective C. Most of them are
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also available in C++. @xref{C++ Extensions,,Extensions to the
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C++ Language}, for extensions that apply @emph{only} to C++.
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@c The only difference between the two versions of this menu is that the
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@c version for clear INTERNALS has an extra node, "Constraints" (which
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@c appears in a separate chapter in the other version of the manual).
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@ifset INTERNALS
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@menu
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* Statement Exprs:: Putting statements and declarations inside expressions.
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* Local Labels:: Labels local to a statement-expression.
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* Labels as Values:: Getting pointers to labels, and computed gotos.
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* Nested Functions:: As in Algol and Pascal, lexical scoping of functions.
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* Constructing Calls:: Dispatching a call to another function.
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* Naming Types:: Giving a name to the type of some expression.
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* Typeof:: @code{typeof}: referring to the type of an expression.
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* Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues.
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* Conditionals:: Omitting the middle operand of a @samp{?:} expression.
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* Long Long:: Double-word integers---@code{long long int}.
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* Complex:: Data types for complex numbers.
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* Zero Length:: Zero-length arrays.
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* Variable Length:: Arrays whose length is computed at run time.
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* Macro Varargs:: Macros with variable number of arguments.
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* Subscripting:: Any array can be subscripted, even if not an lvalue.
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* Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers.
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* Initializers:: Non-constant initializers.
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* Constructors:: Constructor expressions give structures, unions
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or arrays as values.
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* Labeled Elements:: Labeling elements of initializers.
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* Cast to Union:: Casting to union type from any member of the union.
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* Case Ranges:: `case 1 ... 9' and such.
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* Function Attributes:: Declaring that functions have no side effects,
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or that they can never return.
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* Function Prototypes:: Prototype declarations and old-style definitions.
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* C++ Comments:: C++ comments are recognized.
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* Dollar Signs:: Dollar sign is allowed in identifiers.
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* Character Escapes:: @samp{\e} stands for the character @key{ESC}.
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* Variable Attributes:: Specifying attributes of variables.
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* Type Attributes:: Specifying attributes of types.
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* Alignment:: Inquiring about the alignment of a type or variable.
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* Inline:: Defining inline functions (as fast as macros).
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* Extended Asm:: Assembler instructions with C expressions as operands.
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(With them you can define ``built-in'' functions.)
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* Asm Labels:: Specifying the assembler name to use for a C symbol.
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* Explicit Reg Vars:: Defining variables residing in specified registers.
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* Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files.
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* Incomplete Enums:: @code{enum foo;}, with details to follow.
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* Function Names:: Printable strings which are the name of the current
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function.
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* Return Address:: Getting the return or frame address of a function.
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@end menu
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@end ifset
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@ifclear INTERNALS
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@menu
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* Statement Exprs:: Putting statements and declarations inside expressions.
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* Local Labels:: Labels local to a statement-expression.
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* Labels as Values:: Getting pointers to labels, and computed gotos.
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* Nested Functions:: As in Algol and Pascal, lexical scoping of functions.
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* Constructing Calls:: Dispatching a call to another function.
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* Naming Types:: Giving a name to the type of some expression.
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* Typeof:: @code{typeof}: referring to the type of an expression.
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* Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues.
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* Conditionals:: Omitting the middle operand of a @samp{?:} expression.
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* Long Long:: Double-word integers---@code{long long int}.
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* Complex:: Data types for complex numbers.
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* Zero Length:: Zero-length arrays.
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* Variable Length:: Arrays whose length is computed at run time.
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* Macro Varargs:: Macros with variable number of arguments.
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* Subscripting:: Any array can be subscripted, even if not an lvalue.
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* Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers.
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* Initializers:: Non-constant initializers.
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* Constructors:: Constructor expressions give structures, unions
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or arrays as values.
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* Labeled Elements:: Labeling elements of initializers.
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* Cast to Union:: Casting to union type from any member of the union.
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* Case Ranges:: `case 1 ... 9' and such.
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* Function Attributes:: Declaring that functions have no side effects,
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or that they can never return.
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* Function Prototypes:: Prototype declarations and old-style definitions.
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* C++ Comments:: C++ comments are recognized.
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* Dollar Signs:: Dollar sign is allowed in identifiers.
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* Character Escapes:: @samp{\e} stands for the character @key{ESC}.
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* Variable Attributes:: Specifying attributes of variables.
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* Type Attributes:: Specifying attributes of types.
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* Alignment:: Inquiring about the alignment of a type or variable.
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* Inline:: Defining inline functions (as fast as macros).
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* Extended Asm:: Assembler instructions with C expressions as operands.
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(With them you can define ``built-in'' functions.)
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* Constraints:: Constraints for asm operands
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* Asm Labels:: Specifying the assembler name to use for a C symbol.
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* Explicit Reg Vars:: Defining variables residing in specified registers.
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* Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files.
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* Incomplete Enums:: @code{enum foo;}, with details to follow.
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* Function Names:: Printable strings which are the name of the current
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function.
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* Return Address:: Getting the return or frame address of a function.
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@end menu
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@end ifclear
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@node Statement Exprs
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@section Statements and Declarations in Expressions
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@cindex statements inside expressions
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@cindex declarations inside expressions
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@cindex expressions containing statements
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@cindex macros, statements in expressions
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@c the above section title wrapped and causes an underfull hbox.. i
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@c changed it from "within" to "in". --mew 4feb93
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A compound statement enclosed in parentheses may appear as an expression
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in GNU C. This allows you to use loops, switches, and local variables
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within an expression.
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Recall that a compound statement is a sequence of statements surrounded
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by braces; in this construct, parentheses go around the braces. For
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example:
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@example
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(@{ int y = foo (); int z;
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if (y > 0) z = y;
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else z = - y;
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z; @})
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@end example
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@noindent
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is a valid (though slightly more complex than necessary) expression
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for the absolute value of @code{foo ()}.
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The last thing in the compound statement should be an expression
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followed by a semicolon; the value of this subexpression serves as the
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value of the entire construct. (If you use some other kind of statement
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last within the braces, the construct has type @code{void}, and thus
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effectively no value.)
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This feature is especially useful in making macro definitions ``safe'' (so
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that they evaluate each operand exactly once). For example, the
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``maximum'' function is commonly defined as a macro in standard C as
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follows:
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@example
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#define max(a,b) ((a) > (b) ? (a) : (b))
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@end example
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@noindent
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@cindex side effects, macro argument
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But this definition computes either @var{a} or @var{b} twice, with bad
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results if the operand has side effects. In GNU C, if you know the
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type of the operands (here let's assume @code{int}), you can define
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the macro safely as follows:
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@example
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#define maxint(a,b) \
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(@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
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@end example
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Embedded statements are not allowed in constant expressions, such as
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the value of an enumeration constant, the width of a bit field, or
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the initial value of a static variable.
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If you don't know the type of the operand, you can still do this, but you
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must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming
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Types}).
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@node Local Labels
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@section Locally Declared Labels
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@cindex local labels
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@cindex macros, local labels
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Each statement expression is a scope in which @dfn{local labels} can be
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declared. A local label is simply an identifier; you can jump to it
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with an ordinary @code{goto} statement, but only from within the
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statement expression it belongs to.
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A local label declaration looks like this:
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@example
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__label__ @var{label};
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@end example
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@noindent
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or
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@example
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__label__ @var{label1}, @var{label2}, @dots{};
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@end example
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Local label declarations must come at the beginning of the statement
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expression, right after the @samp{(@{}, before any ordinary
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declarations.
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The label declaration defines the label @emph{name}, but does not define
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the label itself. You must do this in the usual way, with
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@code{@var{label}:}, within the statements of the statement expression.
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The local label feature is useful because statement expressions are
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often used in macros. If the macro contains nested loops, a @code{goto}
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can be useful for breaking out of them. However, an ordinary label
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whose scope is the whole function cannot be used: if the macro can be
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expanded several times in one function, the label will be multiply
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defined in that function. A local label avoids this problem. For
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example:
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@example
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#define SEARCH(array, target) \
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(@{ \
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__label__ found; \
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typeof (target) _SEARCH_target = (target); \
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typeof (*(array)) *_SEARCH_array = (array); \
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int i, j; \
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int value; \
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for (i = 0; i < max; i++) \
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for (j = 0; j < max; j++) \
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if (_SEARCH_array[i][j] == _SEARCH_target) \
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@{ value = i; goto found; @} \
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value = -1; \
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found: \
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value; \
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@})
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@end example
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@node Labels as Values
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@section Labels as Values
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@cindex labels as values
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@cindex computed gotos
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@cindex goto with computed label
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@cindex address of a label
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You can get the address of a label defined in the current function
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(or a containing function) with the unary operator @samp{&&}. The
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value has type @code{void *}. This value is a constant and can be used
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wherever a constant of that type is valid. For example:
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@example
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void *ptr;
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@dots{}
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ptr = &&foo;
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@end example
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To use these values, you need to be able to jump to one. This is done
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with the computed goto statement@footnote{The analogous feature in
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Fortran is called an assigned goto, but that name seems inappropriate in
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C, where one can do more than simply store label addresses in label
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variables.}, @code{goto *@var{exp};}. For example,
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@example
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goto *ptr;
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@end example
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@noindent
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Any expression of type @code{void *} is allowed.
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One way of using these constants is in initializing a static array that
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will serve as a jump table:
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@example
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static void *array[] = @{ &&foo, &&bar, &&hack @};
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@end example
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Then you can select a label with indexing, like this:
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@example
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goto *array[i];
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@end example
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@noindent
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Note that this does not check whether the subscript is in bounds---array
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indexing in C never does that.
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Such an array of label values serves a purpose much like that of the
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@code{switch} statement. The @code{switch} statement is cleaner, so
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use that rather than an array unless the problem does not fit a
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@code{switch} statement very well.
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Another use of label values is in an interpreter for threaded code.
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The labels within the interpreter function can be stored in the
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threaded code for super-fast dispatching.
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You can use this mechanism to jump to code in a different function. If
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you do that, totally unpredictable things will happen. The best way to
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avoid this is to store the label address only in automatic variables and
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never pass it as an argument.
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@node Nested Functions
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@section Nested Functions
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@cindex nested functions
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@cindex downward funargs
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@cindex thunks
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A @dfn{nested function} is a function defined inside another function.
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(Nested functions are not supported for GNU C++.) The nested function's
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name is local to the block where it is defined. For example, here we
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define a nested function named @code{square}, and call it twice:
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@example
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@group
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foo (double a, double b)
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@{
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double square (double z) @{ return z * z; @}
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return square (a) + square (b);
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@}
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@end group
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@end example
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The nested function can access all the variables of the containing
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function that are visible at the point of its definition. This is
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called @dfn{lexical scoping}. For example, here we show a nested
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function which uses an inherited variable named @code{offset}:
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@example
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bar (int *array, int offset, int size)
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@{
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int access (int *array, int index)
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@{ return array[index + offset]; @}
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int i;
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@dots{}
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for (i = 0; i < size; i++)
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@dots{} access (array, i) @dots{}
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@}
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@end example
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Nested function definitions are permitted within functions in the places
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where variable definitions are allowed; that is, in any block, before
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the first statement in the block.
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It is possible to call the nested function from outside the scope of its
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name by storing its address or passing the address to another function:
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@example
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hack (int *array, int size)
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@{
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void store (int index, int value)
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@{ array[index] = value; @}
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intermediate (store, size);
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@}
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@end example
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Here, the function @code{intermediate} receives the address of
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@code{store} as an argument. If @code{intermediate} calls @code{store},
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the arguments given to @code{store} are used to store into @code{array}.
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But this technique works only so long as the containing function
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(@code{hack}, in this example) does not exit.
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If you try to call the nested function through its address after the
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containing function has exited, all hell will break loose. If you try
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to call it after a containing scope level has exited, and if it refers
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to some of the variables that are no longer in scope, you may be lucky,
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but it's not wise to take the risk. If, however, the nested function
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does not refer to anything that has gone out of scope, you should be
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safe.
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GNU CC implements taking the address of a nested function using a
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technique called @dfn{trampolines}. A paper describing them is
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available as @samp{http://master.debian.org/~karlheg/Usenix88-lexic.pdf}.
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A nested function can jump to a label inherited from a containing
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function, provided the label was explicitly declared in the containing
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function (@pxref{Local Labels}). Such a jump returns instantly to the
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containing function, exiting the nested function which did the
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@code{goto} and any intermediate functions as well. Here is an example:
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@example
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@group
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bar (int *array, int offset, int size)
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@{
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__label__ failure;
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int access (int *array, int index)
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@{
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if (index > size)
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goto failure;
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return array[index + offset];
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@}
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int i;
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@dots{}
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for (i = 0; i < size; i++)
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@dots{} access (array, i) @dots{}
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@dots{}
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return 0;
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/* @r{Control comes here from @code{access}
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if it detects an error.} */
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failure:
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return -1;
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@}
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@end group
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@end example
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A nested function always has internal linkage. Declaring one with
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@code{extern} is erroneous. If you need to declare the nested function
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before its definition, use @code{auto} (which is otherwise meaningless
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for function declarations).
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@example
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bar (int *array, int offset, int size)
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@{
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__label__ failure;
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auto int access (int *, int);
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@dots{}
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int access (int *array, int index)
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@{
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if (index > size)
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goto failure;
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return array[index + offset];
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@}
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@dots{}
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@}
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@end example
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@node Constructing Calls
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@section Constructing Function Calls
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@cindex constructing calls
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@cindex forwarding calls
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Using the built-in functions described below, you can record
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the arguments a function received, and call another function
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with the same arguments, without knowing the number or types
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of the arguments.
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You can also record the return value of that function call,
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and later return that value, without knowing what data type
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the function tried to return (as long as your caller expects
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that data type).
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@table @code
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@findex __builtin_apply_args
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@item __builtin_apply_args ()
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This built-in function returns a pointer of type @code{void *} to data
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describing how to perform a call with the same arguments as were passed
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to the current function.
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The function saves the arg pointer register, structure value address,
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and all registers that might be used to pass arguments to a function
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into a block of memory allocated on the stack. Then it returns the
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address of that block.
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@findex __builtin_apply
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@item __builtin_apply (@var{function}, @var{arguments}, @var{size})
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This built-in function invokes @var{function} (type @code{void (*)()})
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with a copy of the parameters described by @var{arguments} (type
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@code{void *}) and @var{size} (type @code{int}).
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The value of @var{arguments} should be the value returned by
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@code{__builtin_apply_args}. The argument @var{size} specifies the size
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of the stack argument data, in bytes.
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This function returns a pointer of type @code{void *} to data describing
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how to return whatever value was returned by @var{function}. The data
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is saved in a block of memory allocated on the stack.
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It is not always simple to compute the proper value for @var{size}. The
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value is used by @code{__builtin_apply} to compute the amount of data
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that should be pushed on the stack and copied from the incoming argument
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area.
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@findex __builtin_return
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@item __builtin_return (@var{result})
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This built-in function returns the value described by @var{result} from
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the containing function. You should specify, for @var{result}, a value
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returned by @code{__builtin_apply}.
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@end table
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@node Naming Types
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@section Naming an Expression's Type
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@cindex naming types
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You can give a name to the type of an expression using a @code{typedef}
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declaration with an initializer. Here is how to define @var{name} as a
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type name for the type of @var{exp}:
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@example
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typedef @var{name} = @var{exp};
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@end example
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This is useful in conjunction with the statements-within-expressions
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feature. Here is how the two together can be used to define a safe
|
|
``maximum'' macro that operates on any arithmetic type:
|
|
|
|
@example
|
|
#define max(a,b) \
|
|
(@{typedef _ta = (a), _tb = (b); \
|
|
_ta _a = (a); _tb _b = (b); \
|
|
_a > _b ? _a : _b; @})
|
|
@end example
|
|
|
|
@cindex underscores in variables in macros
|
|
@cindex @samp{_} in variables in macros
|
|
@cindex local variables in macros
|
|
@cindex variables, local, in macros
|
|
@cindex macros, local variables in
|
|
|
|
The reason for using names that start with underscores for the local
|
|
variables is to avoid conflicts with variable names that occur within the
|
|
expressions that are substituted for @code{a} and @code{b}. Eventually we
|
|
hope to design a new form of declaration syntax that allows you to declare
|
|
variables whose scopes start only after their initializers; this will be a
|
|
more reliable way to prevent such conflicts.
|
|
|
|
@node Typeof
|
|
@section Referring to a Type with @code{typeof}
|
|
@findex typeof
|
|
@findex sizeof
|
|
@cindex macros, types of arguments
|
|
|
|
Another way to refer to the type of an expression is with @code{typeof}.
|
|
The syntax of using of this keyword looks like @code{sizeof}, but the
|
|
construct acts semantically like a type name defined with @code{typedef}.
|
|
|
|
There are two ways of writing the argument to @code{typeof}: with an
|
|
expression or with a type. Here is an example with an expression:
|
|
|
|
@example
|
|
typeof (x[0](1))
|
|
@end example
|
|
|
|
@noindent
|
|
This assumes that @code{x} is an array of functions; the type described
|
|
is that of the values of the functions.
|
|
|
|
Here is an example with a typename as the argument:
|
|
|
|
@example
|
|
typeof (int *)
|
|
@end example
|
|
|
|
@noindent
|
|
Here the type described is that of pointers to @code{int}.
|
|
|
|
If you are writing a header file that must work when included in ANSI C
|
|
programs, write @code{__typeof__} instead of @code{typeof}.
|
|
@xref{Alternate Keywords}.
|
|
|
|
A @code{typeof}-construct can be used anywhere a typedef name could be
|
|
used. For example, you can use it in a declaration, in a cast, or inside
|
|
of @code{sizeof} or @code{typeof}.
|
|
|
|
@itemize @bullet
|
|
@item
|
|
This declares @code{y} with the type of what @code{x} points to.
|
|
|
|
@example
|
|
typeof (*x) y;
|
|
@end example
|
|
|
|
@item
|
|
This declares @code{y} as an array of such values.
|
|
|
|
@example
|
|
typeof (*x) y[4];
|
|
@end example
|
|
|
|
@item
|
|
This declares @code{y} as an array of pointers to characters:
|
|
|
|
@example
|
|
typeof (typeof (char *)[4]) y;
|
|
@end example
|
|
|
|
@noindent
|
|
It is equivalent to the following traditional C declaration:
|
|
|
|
@example
|
|
char *y[4];
|
|
@end example
|
|
|
|
To see the meaning of the declaration using @code{typeof}, and why it
|
|
might be a useful way to write, let's rewrite it with these macros:
|
|
|
|
@example
|
|
#define pointer(T) typeof(T *)
|
|
#define array(T, N) typeof(T [N])
|
|
@end example
|
|
|
|
@noindent
|
|
Now the declaration can be rewritten this way:
|
|
|
|
@example
|
|
array (pointer (char), 4) y;
|
|
@end example
|
|
|
|
@noindent
|
|
Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
|
|
pointers to @code{char}.
|
|
@end itemize
|
|
|
|
@node Lvalues
|
|
@section Generalized Lvalues
|
|
@cindex compound expressions as lvalues
|
|
@cindex expressions, compound, as lvalues
|
|
@cindex conditional expressions as lvalues
|
|
@cindex expressions, conditional, as lvalues
|
|
@cindex casts as lvalues
|
|
@cindex generalized lvalues
|
|
@cindex lvalues, generalized
|
|
@cindex extensions, @code{?:}
|
|
@cindex @code{?:} extensions
|
|
Compound expressions, conditional expressions and casts are allowed as
|
|
lvalues provided their operands are lvalues. This means that you can take
|
|
their addresses or store values into them.
|
|
|
|
Standard C++ allows compound expressions and conditional expressions as
|
|
lvalues, and permits casts to reference type, so use of this extension
|
|
is deprecated for C++ code.
|
|
|
|
For example, a compound expression can be assigned, provided the last
|
|
expression in the sequence is an lvalue. These two expressions are
|
|
equivalent:
|
|
|
|
@example
|
|
(a, b) += 5
|
|
a, (b += 5)
|
|
@end example
|
|
|
|
Similarly, the address of the compound expression can be taken. These two
|
|
expressions are equivalent:
|
|
|
|
@example
|
|
&(a, b)
|
|
a, &b
|
|
@end example
|
|
|
|
A conditional expression is a valid lvalue if its type is not void and the
|
|
true and false branches are both valid lvalues. For example, these two
|
|
expressions are equivalent:
|
|
|
|
@example
|
|
(a ? b : c) = 5
|
|
(a ? b = 5 : (c = 5))
|
|
@end example
|
|
|
|
A cast is a valid lvalue if its operand is an lvalue. A simple
|
|
assignment whose left-hand side is a cast works by converting the
|
|
right-hand side first to the specified type, then to the type of the
|
|
inner left-hand side expression. After this is stored, the value is
|
|
converted back to the specified type to become the value of the
|
|
assignment. Thus, if @code{a} has type @code{char *}, the following two
|
|
expressions are equivalent:
|
|
|
|
@example
|
|
(int)a = 5
|
|
(int)(a = (char *)(int)5)
|
|
@end example
|
|
|
|
An assignment-with-arithmetic operation such as @samp{+=} applied to a cast
|
|
performs the arithmetic using the type resulting from the cast, and then
|
|
continues as in the previous case. Therefore, these two expressions are
|
|
equivalent:
|
|
|
|
@example
|
|
(int)a += 5
|
|
(int)(a = (char *)(int) ((int)a + 5))
|
|
@end example
|
|
|
|
You cannot take the address of an lvalue cast, because the use of its
|
|
address would not work out coherently. Suppose that @code{&(int)f} were
|
|
permitted, where @code{f} has type @code{float}. Then the following
|
|
statement would try to store an integer bit-pattern where a floating
|
|
point number belongs:
|
|
|
|
@example
|
|
*&(int)f = 1;
|
|
@end example
|
|
|
|
This is quite different from what @code{(int)f = 1} would do---that
|
|
would convert 1 to floating point and store it. Rather than cause this
|
|
inconsistency, we think it is better to prohibit use of @samp{&} on a cast.
|
|
|
|
If you really do want an @code{int *} pointer with the address of
|
|
@code{f}, you can simply write @code{(int *)&f}.
|
|
|
|
@node Conditionals
|
|
@section Conditionals with Omitted Operands
|
|
@cindex conditional expressions, extensions
|
|
@cindex omitted middle-operands
|
|
@cindex middle-operands, omitted
|
|
@cindex extensions, @code{?:}
|
|
@cindex @code{?:} extensions
|
|
|
|
The middle operand in a conditional expression may be omitted. Then
|
|
if the first operand is nonzero, its value is the value of the conditional
|
|
expression.
|
|
|
|
Therefore, the expression
|
|
|
|
@example
|
|
x ? : y
|
|
@end example
|
|
|
|
@noindent
|
|
has the value of @code{x} if that is nonzero; otherwise, the value of
|
|
@code{y}.
|
|
|
|
This example is perfectly equivalent to
|
|
|
|
@example
|
|
x ? x : y
|
|
@end example
|
|
|
|
@cindex side effect in ?:
|
|
@cindex ?: side effect
|
|
@noindent
|
|
In this simple case, the ability to omit the middle operand is not
|
|
especially useful. When it becomes useful is when the first operand does,
|
|
or may (if it is a macro argument), contain a side effect. Then repeating
|
|
the operand in the middle would perform the side effect twice. Omitting
|
|
the middle operand uses the value already computed without the undesirable
|
|
effects of recomputing it.
|
|
|
|
@node Long Long
|
|
@section Double-Word Integers
|
|
@cindex @code{long long} data types
|
|
@cindex double-word arithmetic
|
|
@cindex multiprecision arithmetic
|
|
|
|
GNU C supports data types for integers that are twice as long as
|
|
@code{int}. Simply write @code{long long int} for a signed integer, or
|
|
@code{unsigned long long int} for an unsigned integer. To make an
|
|
integer constant of type @code{long long int}, add the suffix @code{LL}
|
|
to the integer. To make an integer constant of type @code{unsigned long
|
|
long int}, add the suffix @code{ULL} to the integer.
|
|
|
|
You can use these types in arithmetic like any other integer types.
|
|
Addition, subtraction, and bitwise boolean operations on these types
|
|
are open-coded on all types of machines. Multiplication is open-coded
|
|
if the machine supports fullword-to-doubleword a widening multiply
|
|
instruction. Division and shifts are open-coded only on machines that
|
|
provide special support. The operations that are not open-coded use
|
|
special library routines that come with GNU CC.
|
|
|
|
There may be pitfalls when you use @code{long long} types for function
|
|
arguments, unless you declare function prototypes. If a function
|
|
expects type @code{int} for its argument, and you pass a value of type
|
|
@code{long long int}, confusion will result because the caller and the
|
|
subroutine will disagree about the number of bytes for the argument.
|
|
Likewise, if the function expects @code{long long int} and you pass
|
|
@code{int}. The best way to avoid such problems is to use prototypes.
|
|
|
|
@node Complex
|
|
@section Complex Numbers
|
|
@cindex complex numbers
|
|
|
|
GNU C supports complex data types. You can declare both complex integer
|
|
types and complex floating types, using the keyword @code{__complex__}.
|
|
|
|
For example, @samp{__complex__ double x;} declares @code{x} as a
|
|
variable whose real part and imaginary part are both of type
|
|
@code{double}. @samp{__complex__ short int y;} declares @code{y} to
|
|
have real and imaginary parts of type @code{short int}; this is not
|
|
likely to be useful, but it shows that the set of complex types is
|
|
complete.
|
|
|
|
To write a constant with a complex data type, use the suffix @samp{i} or
|
|
@samp{j} (either one; they are equivalent). For example, @code{2.5fi}
|
|
has type @code{__complex__ float} and @code{3i} has type
|
|
@code{__complex__ int}. Such a constant always has a pure imaginary
|
|
value, but you can form any complex value you like by adding one to a
|
|
real constant.
|
|
|
|
To extract the real part of a complex-valued expression @var{exp}, write
|
|
@code{__real__ @var{exp}}. Likewise, use @code{__imag__} to
|
|
extract the imaginary part.
|
|
|
|
The operator @samp{~} performs complex conjugation when used on a value
|
|
with a complex type.
|
|
|
|
GNU CC can allocate complex automatic variables in a noncontiguous
|
|
fashion; it's even possible for the real part to be in a register while
|
|
the imaginary part is on the stack (or vice-versa). None of the
|
|
supported debugging info formats has a way to represent noncontiguous
|
|
allocation like this, so GNU CC describes a noncontiguous complex
|
|
variable as if it were two separate variables of noncomplex type.
|
|
If the variable's actual name is @code{foo}, the two fictitious
|
|
variables are named @code{foo$real} and @code{foo$imag}. You can
|
|
examine and set these two fictitious variables with your debugger.
|
|
|
|
A future version of GDB will know how to recognize such pairs and treat
|
|
them as a single variable with a complex type.
|
|
|
|
@node Zero Length
|
|
@section Arrays of Length Zero
|
|
@cindex arrays of length zero
|
|
@cindex zero-length arrays
|
|
@cindex length-zero arrays
|
|
|
|
Zero-length arrays are allowed in GNU C. They are very useful as the last
|
|
element of a structure which is really a header for a variable-length
|
|
object:
|
|
|
|
@example
|
|
struct line @{
|
|
int length;
|
|
char contents[0];
|
|
@};
|
|
|
|
@{
|
|
struct line *thisline = (struct line *)
|
|
malloc (sizeof (struct line) + this_length);
|
|
thisline->length = this_length;
|
|
@}
|
|
@end example
|
|
|
|
In standard C, you would have to give @code{contents} a length of 1, which
|
|
means either you waste space or complicate the argument to @code{malloc}.
|
|
|
|
@node Variable Length
|
|
@section Arrays of Variable Length
|
|
@cindex variable-length arrays
|
|
@cindex arrays of variable length
|
|
|
|
Variable-length automatic arrays are allowed in GNU C. These arrays are
|
|
declared like any other automatic arrays, but with a length that is not
|
|
a constant expression. The storage is allocated at the point of
|
|
declaration and deallocated when the brace-level is exited. For
|
|
example:
|
|
|
|
@example
|
|
FILE *
|
|
concat_fopen (char *s1, char *s2, char *mode)
|
|
@{
|
|
char str[strlen (s1) + strlen (s2) + 1];
|
|
strcpy (str, s1);
|
|
strcat (str, s2);
|
|
return fopen (str, mode);
|
|
@}
|
|
@end example
|
|
|
|
@cindex scope of a variable length array
|
|
@cindex variable-length array scope
|
|
@cindex deallocating variable length arrays
|
|
Jumping or breaking out of the scope of the array name deallocates the
|
|
storage. Jumping into the scope is not allowed; you get an error
|
|
message for it.
|
|
|
|
@cindex @code{alloca} vs variable-length arrays
|
|
You can use the function @code{alloca} to get an effect much like
|
|
variable-length arrays. The function @code{alloca} is available in
|
|
many other C implementations (but not in all). On the other hand,
|
|
variable-length arrays are more elegant.
|
|
|
|
There are other differences between these two methods. Space allocated
|
|
with @code{alloca} exists until the containing @emph{function} returns.
|
|
The space for a variable-length array is deallocated as soon as the array
|
|
name's scope ends. (If you use both variable-length arrays and
|
|
@code{alloca} in the same function, deallocation of a variable-length array
|
|
will also deallocate anything more recently allocated with @code{alloca}.)
|
|
|
|
You can also use variable-length arrays as arguments to functions:
|
|
|
|
@example
|
|
struct entry
|
|
tester (int len, char data[len][len])
|
|
@{
|
|
@dots{}
|
|
@}
|
|
@end example
|
|
|
|
The length of an array is computed once when the storage is allocated
|
|
and is remembered for the scope of the array in case you access it with
|
|
@code{sizeof}.
|
|
|
|
If you want to pass the array first and the length afterward, you can
|
|
use a forward declaration in the parameter list---another GNU extension.
|
|
|
|
@example
|
|
struct entry
|
|
tester (int len; char data[len][len], int len)
|
|
@{
|
|
@dots{}
|
|
@}
|
|
@end example
|
|
|
|
@cindex parameter forward declaration
|
|
The @samp{int len} before the semicolon is a @dfn{parameter forward
|
|
declaration}, and it serves the purpose of making the name @code{len}
|
|
known when the declaration of @code{data} is parsed.
|
|
|
|
You can write any number of such parameter forward declarations in the
|
|
parameter list. They can be separated by commas or semicolons, but the
|
|
last one must end with a semicolon, which is followed by the ``real''
|
|
parameter declarations. Each forward declaration must match a ``real''
|
|
declaration in parameter name and data type.
|
|
|
|
@node Macro Varargs
|
|
@section Macros with Variable Numbers of Arguments
|
|
@cindex variable number of arguments
|
|
@cindex macro with variable arguments
|
|
@cindex rest argument (in macro)
|
|
|
|
In GNU C, a macro can accept a variable number of arguments, much as a
|
|
function can. The syntax for defining the macro looks much like that
|
|
used for a function. Here is an example:
|
|
|
|
@example
|
|
#define eprintf(format, args...) \
|
|
fprintf (stderr, format , ## args)
|
|
@end example
|
|
|
|
Here @code{args} is a @dfn{rest argument}: it takes in zero or more
|
|
arguments, as many as the call contains. All of them plus the commas
|
|
between them form the value of @code{args}, which is substituted into
|
|
the macro body where @code{args} is used. Thus, we have this expansion:
|
|
|
|
@example
|
|
eprintf ("%s:%d: ", input_file_name, line_number)
|
|
@expansion{}
|
|
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
|
|
@end example
|
|
|
|
@noindent
|
|
Note that the comma after the string constant comes from the definition
|
|
of @code{eprintf}, whereas the last comma comes from the value of
|
|
@code{args}.
|
|
|
|
The reason for using @samp{##} is to handle the case when @code{args}
|
|
matches no arguments at all. In this case, @code{args} has an empty
|
|
value. In this case, the second comma in the definition becomes an
|
|
embarrassment: if it got through to the expansion of the macro, we would
|
|
get something like this:
|
|
|
|
@example
|
|
fprintf (stderr, "success!\n" , )
|
|
@end example
|
|
|
|
@noindent
|
|
which is invalid C syntax. @samp{##} gets rid of the comma, so we get
|
|
the following instead:
|
|
|
|
@example
|
|
fprintf (stderr, "success!\n")
|
|
@end example
|
|
|
|
This is a special feature of the GNU C preprocessor: @samp{##} before a
|
|
rest argument that is empty discards the preceding sequence of
|
|
non-whitespace characters from the macro definition. (If another macro
|
|
argument precedes, none of it is discarded.)
|
|
|
|
It might be better to discard the last preprocessor token instead of the
|
|
last preceding sequence of non-whitespace characters; in fact, we may
|
|
someday change this feature to do so. We advise you to write the macro
|
|
definition so that the preceding sequence of non-whitespace characters
|
|
is just a single token, so that the meaning will not change if we change
|
|
the definition of this feature.
|
|
|
|
@node Subscripting
|
|
@section Non-Lvalue Arrays May Have Subscripts
|
|
@cindex subscripting
|
|
@cindex arrays, non-lvalue
|
|
|
|
@cindex subscripting and function values
|
|
Subscripting is allowed on arrays that are not lvalues, even though the
|
|
unary @samp{&} operator is not. For example, this is valid in GNU C though
|
|
not valid in other C dialects:
|
|
|
|
@example
|
|
@group
|
|
struct foo @{int a[4];@};
|
|
|
|
struct foo f();
|
|
|
|
bar (int index)
|
|
@{
|
|
return f().a[index];
|
|
@}
|
|
@end group
|
|
@end example
|
|
|
|
@node Pointer Arith
|
|
@section Arithmetic on @code{void}- and Function-Pointers
|
|
@cindex void pointers, arithmetic
|
|
@cindex void, size of pointer to
|
|
@cindex function pointers, arithmetic
|
|
@cindex function, size of pointer to
|
|
|
|
In GNU C, addition and subtraction operations are supported on pointers to
|
|
@code{void} and on pointers to functions. This is done by treating the
|
|
size of a @code{void} or of a function as 1.
|
|
|
|
A consequence of this is that @code{sizeof} is also allowed on @code{void}
|
|
and on function types, and returns 1.
|
|
|
|
The option @samp{-Wpointer-arith} requests a warning if these extensions
|
|
are used.
|
|
|
|
@node Initializers
|
|
@section Non-Constant Initializers
|
|
@cindex initializers, non-constant
|
|
@cindex non-constant initializers
|
|
|
|
As in standard C++, the elements of an aggregate initializer for an
|
|
automatic variable are not required to be constant expressions in GNU C.
|
|
Here is an example of an initializer with run-time varying elements:
|
|
|
|
@example
|
|
foo (float f, float g)
|
|
@{
|
|
float beat_freqs[2] = @{ f-g, f+g @};
|
|
@dots{}
|
|
@}
|
|
@end example
|
|
|
|
@node Constructors
|
|
@section Constructor Expressions
|
|
@cindex constructor expressions
|
|
@cindex initializations in expressions
|
|
@cindex structures, constructor expression
|
|
@cindex expressions, constructor
|
|
|
|
GNU C supports constructor expressions. A constructor looks like
|
|
a cast containing an initializer. Its value is an object of the
|
|
type specified in the cast, containing the elements specified in
|
|
the initializer.
|
|
|
|
Usually, the specified type is a structure. Assume that
|
|
@code{struct foo} and @code{structure} are declared as shown:
|
|
|
|
@example
|
|
struct foo @{int a; char b[2];@} structure;
|
|
@end example
|
|
|
|
@noindent
|
|
Here is an example of constructing a @code{struct foo} with a constructor:
|
|
|
|
@example
|
|
structure = ((struct foo) @{x + y, 'a', 0@});
|
|
@end example
|
|
|
|
@noindent
|
|
This is equivalent to writing the following:
|
|
|
|
@example
|
|
@{
|
|
struct foo temp = @{x + y, 'a', 0@};
|
|
structure = temp;
|
|
@}
|
|
@end example
|
|
|
|
You can also construct an array. If all the elements of the constructor
|
|
are (made up of) simple constant expressions, suitable for use in
|
|
initializers, then the constructor is an lvalue and can be coerced to a
|
|
pointer to its first element, as shown here:
|
|
|
|
@example
|
|
char **foo = (char *[]) @{ "x", "y", "z" @};
|
|
@end example
|
|
|
|
Array constructors whose elements are not simple constants are
|
|
not very useful, because the constructor is not an lvalue. There
|
|
are only two valid ways to use it: to subscript it, or initialize
|
|
an array variable with it. The former is probably slower than a
|
|
@code{switch} statement, while the latter does the same thing an
|
|
ordinary C initializer would do. Here is an example of
|
|
subscripting an array constructor:
|
|
|
|
@example
|
|
output = ((int[]) @{ 2, x, 28 @}) [input];
|
|
@end example
|
|
|
|
Constructor expressions for scalar types and union types are is
|
|
also allowed, but then the constructor expression is equivalent
|
|
to a cast.
|
|
|
|
@node Labeled Elements
|
|
@section Labeled Elements in Initializers
|
|
@cindex initializers with labeled elements
|
|
@cindex labeled elements in initializers
|
|
@cindex case labels in initializers
|
|
|
|
Standard C requires the elements of an initializer to appear in a fixed
|
|
order, the same as the order of the elements in the array or structure
|
|
being initialized.
|
|
|
|
In GNU C you can give the elements in any order, specifying the array
|
|
indices or structure field names they apply to. This extension is not
|
|
implemented in GNU C++.
|
|
|
|
To specify an array index, write @samp{[@var{index}]} or
|
|
@samp{[@var{index}] =} before the element value. For example,
|
|
|
|
@example
|
|
int a[6] = @{ [4] 29, [2] = 15 @};
|
|
@end example
|
|
|
|
@noindent
|
|
is equivalent to
|
|
|
|
@example
|
|
int a[6] = @{ 0, 0, 15, 0, 29, 0 @};
|
|
@end example
|
|
|
|
@noindent
|
|
The index values must be constant expressions, even if the array being
|
|
initialized is automatic.
|
|
|
|
To initialize a range of elements to the same value, write
|
|
@samp{[@var{first} ... @var{last}] = @var{value}}. For example,
|
|
|
|
@example
|
|
int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @};
|
|
@end example
|
|
|
|
@noindent
|
|
Note that the length of the array is the highest value specified
|
|
plus one.
|
|
|
|
In a structure initializer, specify the name of a field to initialize
|
|
with @samp{@var{fieldname}:} before the element value. For example,
|
|
given the following structure,
|
|
|
|
@example
|
|
struct point @{ int x, y; @};
|
|
@end example
|
|
|
|
@noindent
|
|
the following initialization
|
|
|
|
@example
|
|
struct point p = @{ y: yvalue, x: xvalue @};
|
|
@end example
|
|
|
|
@noindent
|
|
is equivalent to
|
|
|
|
@example
|
|
struct point p = @{ xvalue, yvalue @};
|
|
@end example
|
|
|
|
Another syntax which has the same meaning is @samp{.@var{fieldname} =}.,
|
|
as shown here:
|
|
|
|
@example
|
|
struct point p = @{ .y = yvalue, .x = xvalue @};
|
|
@end example
|
|
|
|
You can also use an element label (with either the colon syntax or the
|
|
period-equal syntax) when initializing a union, to specify which element
|
|
of the union should be used. For example,
|
|
|
|
@example
|
|
union foo @{ int i; double d; @};
|
|
|
|
union foo f = @{ d: 4 @};
|
|
@end example
|
|
|
|
@noindent
|
|
will convert 4 to a @code{double} to store it in the union using
|
|
the second element. By contrast, casting 4 to type @code{union foo}
|
|
would store it into the union as the integer @code{i}, since it is
|
|
an integer. (@xref{Cast to Union}.)
|
|
|
|
You can combine this technique of naming elements with ordinary C
|
|
initialization of successive elements. Each initializer element that
|
|
does not have a label applies to the next consecutive element of the
|
|
array or structure. For example,
|
|
|
|
@example
|
|
int a[6] = @{ [1] = v1, v2, [4] = v4 @};
|
|
@end example
|
|
|
|
@noindent
|
|
is equivalent to
|
|
|
|
@example
|
|
int a[6] = @{ 0, v1, v2, 0, v4, 0 @};
|
|
@end example
|
|
|
|
Labeling the elements of an array initializer is especially useful
|
|
when the indices are characters or belong to an @code{enum} type.
|
|
For example:
|
|
|
|
@example
|
|
int whitespace[256]
|
|
= @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1,
|
|
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @};
|
|
@end example
|
|
|
|
@node Case Ranges
|
|
@section Case Ranges
|
|
@cindex case ranges
|
|
@cindex ranges in case statements
|
|
|
|
You can specify a range of consecutive values in a single @code{case} label,
|
|
like this:
|
|
|
|
@example
|
|
case @var{low} ... @var{high}:
|
|
@end example
|
|
|
|
@noindent
|
|
This has the same effect as the proper number of individual @code{case}
|
|
labels, one for each integer value from @var{low} to @var{high}, inclusive.
|
|
|
|
This feature is especially useful for ranges of ASCII character codes:
|
|
|
|
@example
|
|
case 'A' ... 'Z':
|
|
@end example
|
|
|
|
@strong{Be careful:} Write spaces around the @code{...}, for otherwise
|
|
it may be parsed wrong when you use it with integer values. For example,
|
|
write this:
|
|
|
|
@example
|
|
case 1 ... 5:
|
|
@end example
|
|
|
|
@noindent
|
|
rather than this:
|
|
|
|
@example
|
|
case 1...5:
|
|
@end example
|
|
|
|
@node Cast to Union
|
|
@section Cast to a Union Type
|
|
@cindex cast to a union
|
|
@cindex union, casting to a
|
|
|
|
A cast to union type is similar to other casts, except that the type
|
|
specified is a union type. You can specify the type either with
|
|
@code{union @var{tag}} or with a typedef name. A cast to union is actually
|
|
a constructor though, not a cast, and hence does not yield an lvalue like
|
|
normal casts. (@xref{Constructors}.)
|
|
|
|
The types that may be cast to the union type are those of the members
|
|
of the union. Thus, given the following union and variables:
|
|
|
|
@example
|
|
union foo @{ int i; double d; @};
|
|
int x;
|
|
double y;
|
|
@end example
|
|
|
|
@noindent
|
|
both @code{x} and @code{y} can be cast to type @code{union} foo.
|
|
|
|
Using the cast as the right-hand side of an assignment to a variable of
|
|
union type is equivalent to storing in a member of the union:
|
|
|
|
@example
|
|
union foo u;
|
|
@dots{}
|
|
u = (union foo) x @equiv{} u.i = x
|
|
u = (union foo) y @equiv{} u.d = y
|
|
@end example
|
|
|
|
You can also use the union cast as a function argument:
|
|
|
|
@example
|
|
void hack (union foo);
|
|
@dots{}
|
|
hack ((union foo) x);
|
|
@end example
|
|
|
|
@node Function Attributes
|
|
@section Declaring Attributes of Functions
|
|
@cindex function attributes
|
|
@cindex declaring attributes of functions
|
|
@cindex functions that never return
|
|
@cindex functions that have no side effects
|
|
@cindex functions in arbitrary sections
|
|
@cindex @code{volatile} applied to function
|
|
@cindex @code{const} applied to function
|
|
@cindex functions with @code{printf}, @code{scanf} or @code{strftime} style arguments
|
|
@cindex functions that are passed arguments in registers on the 386
|
|
@cindex functions that pop the argument stack on the 386
|
|
@cindex functions that do not pop the argument stack on the 386
|
|
|
|
In GNU C, you declare certain things about functions called in your program
|
|
which help the compiler optimize function calls and check your code more
|
|
carefully.
|
|
|
|
The keyword @code{__attribute__} allows you to specify special
|
|
attributes when making a declaration. This keyword is followed by an
|
|
attribute specification inside double parentheses. Eight attributes,
|
|
@code{noreturn}, @code{const}, @code{format}, @code{section},
|
|
@code{constructor}, @code{destructor}, @code{unused} and @code{weak} are
|
|
currently defined for functions. Other attributes, including
|
|
@code{section} are supported for variables declarations (@pxref{Variable
|
|
Attributes}) and for types (@pxref{Type Attributes}).
|
|
|
|
You may also specify attributes with @samp{__} preceding and following
|
|
each keyword. This allows you to use them in header files without
|
|
being concerned about a possible macro of the same name. For example,
|
|
you may use @code{__noreturn__} instead of @code{noreturn}.
|
|
|
|
@table @code
|
|
@cindex @code{noreturn} function attribute
|
|
@item noreturn
|
|
A few standard library functions, such as @code{abort} and @code{exit},
|
|
cannot return. GNU CC knows this automatically. Some programs define
|
|
their own functions that never return. You can declare them
|
|
@code{noreturn} to tell the compiler this fact. For example,
|
|
|
|
@smallexample
|
|
void fatal () __attribute__ ((noreturn));
|
|
|
|
void
|
|
fatal (@dots{})
|
|
@{
|
|
@dots{} /* @r{Print error message.} */ @dots{}
|
|
exit (1);
|
|
@}
|
|
@end smallexample
|
|
|
|
The @code{noreturn} keyword tells the compiler to assume that
|
|
@code{fatal} cannot return. It can then optimize without regard to what
|
|
would happen if @code{fatal} ever did return. This makes slightly
|
|
better code. More importantly, it helps avoid spurious warnings of
|
|
uninitialized variables.
|
|
|
|
Do not assume that registers saved by the calling function are
|
|
restored before calling the @code{noreturn} function.
|
|
|
|
It does not make sense for a @code{noreturn} function to have a return
|
|
type other than @code{void}.
|
|
|
|
The attribute @code{noreturn} is not implemented in GNU C versions
|
|
earlier than 2.5. An alternative way to declare that a function does
|
|
not return, which works in the current version and in some older
|
|
versions, is as follows:
|
|
|
|
@smallexample
|
|
typedef void voidfn ();
|
|
|
|
volatile voidfn fatal;
|
|
@end smallexample
|
|
|
|
@cindex @code{const} function attribute
|
|
@item const
|
|
Many functions do not examine any values except their arguments, and
|
|
have no effects except the return value. Such a function can be subject
|
|
to common subexpression elimination and loop optimization just as an
|
|
arithmetic operator would be. These functions should be declared
|
|
with the attribute @code{const}. For example,
|
|
|
|
@smallexample
|
|
int square (int) __attribute__ ((const));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
says that the hypothetical function @code{square} is safe to call
|
|
fewer times than the program says.
|
|
|
|
The attribute @code{const} is not implemented in GNU C versions earlier
|
|
than 2.5. An alternative way to declare that a function has no side
|
|
effects, which works in the current version and in some older versions,
|
|
is as follows:
|
|
|
|
@smallexample
|
|
typedef int intfn ();
|
|
|
|
extern const intfn square;
|
|
@end smallexample
|
|
|
|
This approach does not work in GNU C++ from 2.6.0 on, since the language
|
|
specifies that the @samp{const} must be attached to the return value.
|
|
|
|
@cindex pointer arguments
|
|
Note that a function that has pointer arguments and examines the data
|
|
pointed to must @emph{not} be declared @code{const}. Likewise, a
|
|
function that calls a non-@code{const} function usually must not be
|
|
@code{const}. It does not make sense for a @code{const} function to
|
|
return @code{void}.
|
|
|
|
@item format (@var{archetype}, @var{string-index}, @var{first-to-check})
|
|
@cindex @code{format} function attribute
|
|
The @code{format} attribute specifies that a function takes @code{printf},
|
|
@code{scanf}, or @code{strftime} style arguments which should be type-checked
|
|
against a format string. For example, the declaration:
|
|
|
|
@smallexample
|
|
extern int
|
|
my_printf (void *my_object, const char *my_format, ...)
|
|
__attribute__ ((format (printf, 2, 3)));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
causes the compiler to check the arguments in calls to @code{my_printf}
|
|
for consistency with the @code{printf} style format string argument
|
|
@code{my_format}.
|
|
|
|
The parameter @var{archetype} determines how the format string is
|
|
interpreted, and should be either @code{printf}, @code{printf0}, @code{scanf},
|
|
or @code{strftime}. @code{printf0} allows the format string to be a null
|
|
pointer, while @code{printf} does not. The
|
|
parameter @var{string-index} specifies which argument is the format
|
|
string argument (starting from 1), while @var{first-to-check} is the
|
|
number of the first argument to check against the format string. For
|
|
functions where the arguments are not available to be checked (such as
|
|
@code{vprintf}), specify the third parameter as zero. In this case the
|
|
compiler only checks the format string for consistency.
|
|
|
|
In the example above, the format string (@code{my_format}) is the second
|
|
argument of the function @code{my_print}, and the arguments to check
|
|
start with the third argument, so the correct parameters for the format
|
|
attribute are 2 and 3.
|
|
|
|
The @code{format} attribute allows you to identify your own functions
|
|
which take format strings as arguments, so that GNU CC can check the
|
|
calls to these functions for errors. The compiler always checks formats
|
|
for the ANSI library functions @code{printf}, @code{fprintf},
|
|
@code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime},
|
|
@code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
|
|
warnings are requested (using @samp{-Wformat}), so there is no need to
|
|
modify the header file @file{stdio.h}.
|
|
|
|
@item format_arg (@var{string-index})
|
|
@cindex @code{format_arg} function attribute
|
|
The @code{format_arg} attribute specifies that a function takes
|
|
@code{printf} or @code{scanf} style arguments, modifies it (for example,
|
|
to translate it into another language), and passes it to a @code{printf}
|
|
or @code{scanf} style function. For example, the declaration:
|
|
|
|
@smallexample
|
|
extern char *
|
|
my_dgettext (char *my_domain, const char *my_format)
|
|
__attribute__ ((format_arg (2)));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
causes the compiler to check the arguments in calls to
|
|
@code{my_dgettext} whose result is passed to a @code{printf},
|
|
@code{scanf}, or @code{strftime} type function for consistency with the
|
|
@code{printf} style format string argument @code{my_format}.
|
|
|
|
The parameter @var{string-index} specifies which argument is the format
|
|
string argument (starting from 1).
|
|
|
|
The @code{format-arg} attribute allows you to identify your own
|
|
functions which modify format strings, so that GNU CC can check the
|
|
calls to @code{printf}, @code{scanf}, or @code{strftime} function whose
|
|
operands are a call to one of your own function. The compiler always
|
|
treats @code{gettext}, @code{dgettext}, and @code{dcgettext} in this
|
|
manner.
|
|
|
|
@item section ("section-name")
|
|
@cindex @code{section} function attribute
|
|
Normally, the compiler places the code it generates in the @code{text} section.
|
|
Sometimes, however, you need additional sections, or you need certain
|
|
particular functions to appear in special sections. The @code{section}
|
|
attribute specifies that a function lives in a particular section.
|
|
For example, the declaration:
|
|
|
|
@smallexample
|
|
extern void foobar (void) __attribute__ ((section ("bar")));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
puts the function @code{foobar} in the @code{bar} section.
|
|
|
|
Some file formats do not support arbitrary sections so the @code{section}
|
|
attribute is not available on all platforms.
|
|
If you need to map the entire contents of a module to a particular
|
|
section, consider using the facilities of the linker instead.
|
|
|
|
@item constructor
|
|
@itemx destructor
|
|
@cindex @code{constructor} function attribute
|
|
@cindex @code{destructor} function attribute
|
|
The @code{constructor} attribute causes the function to be called
|
|
automatically before execution enters @code{main ()}. Similarly, the
|
|
@code{destructor} attribute causes the function to be called
|
|
automatically after @code{main ()} has completed or @code{exit ()} has
|
|
been called. Functions with these attributes are useful for
|
|
initializing data that will be used implicitly during the execution of
|
|
the program.
|
|
|
|
These attributes are not currently implemented for Objective C.
|
|
|
|
@item unused
|
|
This attribute, attached to a function, means that the function is meant
|
|
to be possibly unused. GNU CC will not produce a warning for this
|
|
function. GNU C++ does not currently support this attribute as
|
|
definitions without parameters are valid in C++.
|
|
|
|
@item weak
|
|
@cindex @code{weak} attribute
|
|
The @code{weak} attribute causes the declaration to be emitted as a weak
|
|
symbol rather than a global. This is primarily useful in defining
|
|
library functions which can be overridden in user code, though it can
|
|
also be used with non-function declarations. Weak symbols are supported
|
|
for ELF targets, and also for a.out targets when using the GNU assembler
|
|
and linker.
|
|
|
|
@item alias ("target")
|
|
@cindex @code{alias} attribute
|
|
The @code{alias} attribute causes the declaration to be emitted as an
|
|
alias for another symbol, which must be specified. For instance,
|
|
|
|
@smallexample
|
|
void __f () @{ /* do something */; @}
|
|
void f () __attribute__ ((weak, alias ("__f")));
|
|
@end smallexample
|
|
|
|
declares @samp{f} to be a weak alias for @samp{__f}. In C++, the
|
|
mangled name for the target must be used.
|
|
|
|
Not all target machines support this attribute.
|
|
|
|
@item regparm (@var{number})
|
|
@cindex functions that are passed arguments in registers on the 386
|
|
On the Intel 386, the @code{regparm} attribute causes the compiler to
|
|
pass up to @var{number} integer arguments in registers @var{EAX},
|
|
@var{EDX}, and @var{ECX} instead of on the stack. Functions that take a
|
|
variable number of arguments will continue to be passed all of their
|
|
arguments on the stack.
|
|
|
|
@item stdcall
|
|
@cindex functions that pop the argument stack on the 386
|
|
On the Intel 386, the @code{stdcall} attribute causes the compiler to
|
|
assume that the called function will pop off the stack space used to
|
|
pass arguments, unless it takes a variable number of arguments.
|
|
|
|
The PowerPC compiler for Windows NT currently ignores the @code{stdcall}
|
|
attribute.
|
|
|
|
@item cdecl
|
|
@cindex functions that do pop the argument stack on the 386
|
|
On the Intel 386, the @code{cdecl} attribute causes the compiler to
|
|
assume that the calling function will pop off the stack space used to
|
|
pass arguments. This is
|
|
useful to override the effects of the @samp{-mrtd} switch.
|
|
|
|
The PowerPC compiler for Windows NT currently ignores the @code{cdecl}
|
|
attribute.
|
|
|
|
@item longcall
|
|
@cindex functions called via pointer on the RS/6000 and PowerPC
|
|
On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
|
|
compiler to always call the function via a pointer, so that functions
|
|
which reside further than 64 megabytes (67,108,864 bytes) from the
|
|
current location can be called.
|
|
|
|
@item dllimport
|
|
@cindex functions which are imported from a dll on PowerPC Windows NT
|
|
On the PowerPC running Windows NT, the @code{dllimport} attribute causes
|
|
the compiler to call the function via a global pointer to the function
|
|
pointer that is set up by the Windows NT dll library. The pointer name
|
|
is formed by combining @code{__imp_} and the function name.
|
|
|
|
@item dllexport
|
|
@cindex functions which are exported from a dll on PowerPC Windows NT
|
|
On the PowerPC running Windows NT, the @code{dllexport} attribute causes
|
|
the compiler to provide a global pointer to the function pointer, so
|
|
that it can be called with the @code{dllimport} attribute. The pointer
|
|
name is formed by combining @code{__imp_} and the function name.
|
|
|
|
@item exception (@var{except-func} [, @var{except-arg}])
|
|
@cindex functions which specify exception handling on PowerPC Windows NT
|
|
On the PowerPC running Windows NT, the @code{exception} attribute causes
|
|
the compiler to modify the structured exception table entry it emits for
|
|
the declared function. The string or identifier @var{except-func} is
|
|
placed in the third entry of the structured exception table. It
|
|
represents a function, which is called by the exception handling
|
|
mechanism if an exception occurs. If it was specified, the string or
|
|
identifier @var{except-arg} is placed in the fourth entry of the
|
|
structured exception table.
|
|
|
|
@item function_vector
|
|
@cindex calling functions through the function vector on the H8/300 processors
|
|
Use this option on the H8/300 and H8/300H to indicate that the specified
|
|
function should be called through the function vector. Calling a
|
|
function through the function vector will reduce code size, however;
|
|
the function vector has a limited size (maximum 128 entries on the H8/300
|
|
and 64 entries on the H8/300H) and shares space with the interrupt vector.
|
|
|
|
You must use GAS and GLD from GNU binutils version 2.7 or later for
|
|
this option to work correctly.
|
|
|
|
@item interrupt_handler
|
|
@cindex interrupt handler functions on the H8/300 processors
|
|
Use this option on the H8/300 and H8/300H to indicate that the specified
|
|
function is an interrupt handler. The compiler will generate function
|
|
entry and exit sequences suitable for use in an interrupt handler when this
|
|
attribute is present.
|
|
|
|
@item eightbit_data
|
|
@cindex eight bit data on the H8/300 and H8/300H
|
|
Use this option on the H8/300 and H8/300H to indicate that the specified
|
|
variable should be placed into the eight bit data section.
|
|
The compiler will generate more efficient code for certain operations
|
|
on data in the eight bit data area. Note the eight bit data area is limited to
|
|
256 bytes of data.
|
|
|
|
You must use GAS and GLD from GNU binutils version 2.7 or later for
|
|
this option to work correctly.
|
|
|
|
@item tiny_data
|
|
@cindex tiny data section on the H8/300H
|
|
Use this option on the H8/300H to indicate that the specified
|
|
variable should be placed into the tiny data section.
|
|
The compiler will generate more efficient code for loads and stores
|
|
on data in the tiny data section. Note the tiny data area is limited to
|
|
slightly under 32kbytes of data.
|
|
|
|
@item interrupt
|
|
@cindex interrupt handlers on the M32R/D
|
|
Use this option on the M32R/D to indicate that the specified
|
|
function is an interrupt handler. The compiler will generate function
|
|
entry and exit sequences suitable for use in an interrupt handler when this
|
|
attribute is present.
|
|
|
|
@item model (@var{model-name})
|
|
@cindex function addressability on the M32R/D
|
|
Use this attribute on the M32R/D to set the addressability of an object,
|
|
and the code generated for a function.
|
|
The identifier @var{model-name} is one of @code{small}, @code{medium},
|
|
or @code{large}, representing each of the code models.
|
|
|
|
Small model objects live in the lower 16MB of memory (so that their
|
|
addresses can be loaded with the @code{ld24} instruction), and are
|
|
callable with the @code{bl} instruction.
|
|
|
|
Medium model objects may live anywhere in the 32 bit address space (the
|
|
compiler will generate @code{seth/add3} instructions to load their addresses),
|
|
and are callable with the @code{bl} instruction.
|
|
|
|
Large model objects may live anywhere in the 32 bit address space (the
|
|
compiler will generate @code{seth/add3} instructions to load their addresses),
|
|
and may not be reachable with the @code{bl} instruction (the compiler will
|
|
generate the much slower @code{seth/add3/jl} instruction sequence).
|
|
|
|
@end table
|
|
|
|
You can specify multiple attributes in a declaration by separating them
|
|
by commas within the double parentheses or by immediately following an
|
|
attribute declaration with another attribute declaration.
|
|
|
|
@cindex @code{#pragma}, reason for not using
|
|
@cindex pragma, reason for not using
|
|
Some people object to the @code{__attribute__} feature, suggesting that ANSI C's
|
|
@code{#pragma} should be used instead. There are two reasons for not
|
|
doing this.
|
|
|
|
@enumerate
|
|
@item
|
|
It is impossible to generate @code{#pragma} commands from a macro.
|
|
|
|
@item
|
|
There is no telling what the same @code{#pragma} might mean in another
|
|
compiler.
|
|
@end enumerate
|
|
|
|
These two reasons apply to almost any application that might be proposed
|
|
for @code{#pragma}. It is basically a mistake to use @code{#pragma} for
|
|
@emph{anything}.
|
|
|
|
@node Function Prototypes
|
|
@section Prototypes and Old-Style Function Definitions
|
|
@cindex function prototype declarations
|
|
@cindex old-style function definitions
|
|
@cindex promotion of formal parameters
|
|
|
|
GNU C extends ANSI C to allow a function prototype to override a later
|
|
old-style non-prototype definition. Consider the following example:
|
|
|
|
@example
|
|
/* @r{Use prototypes unless the compiler is old-fashioned.} */
|
|
#ifdef __STDC__
|
|
#define P(x) x
|
|
#else
|
|
#define P(x) ()
|
|
#endif
|
|
|
|
/* @r{Prototype function declaration.} */
|
|
int isroot P((uid_t));
|
|
|
|
/* @r{Old-style function definition.} */
|
|
int
|
|
isroot (x) /* ??? lossage here ??? */
|
|
uid_t x;
|
|
@{
|
|
return x == 0;
|
|
@}
|
|
@end example
|
|
|
|
Suppose the type @code{uid_t} happens to be @code{short}. ANSI C does
|
|
not allow this example, because subword arguments in old-style
|
|
non-prototype definitions are promoted. Therefore in this example the
|
|
function definition's argument is really an @code{int}, which does not
|
|
match the prototype argument type of @code{short}.
|
|
|
|
This restriction of ANSI C makes it hard to write code that is portable
|
|
to traditional C compilers, because the programmer does not know
|
|
whether the @code{uid_t} type is @code{short}, @code{int}, or
|
|
@code{long}. Therefore, in cases like these GNU C allows a prototype
|
|
to override a later old-style definition. More precisely, in GNU C, a
|
|
function prototype argument type overrides the argument type specified
|
|
by a later old-style definition if the former type is the same as the
|
|
latter type before promotion. Thus in GNU C the above example is
|
|
equivalent to the following:
|
|
|
|
@example
|
|
int isroot (uid_t);
|
|
|
|
int
|
|
isroot (uid_t x)
|
|
@{
|
|
return x == 0;
|
|
@}
|
|
@end example
|
|
|
|
GNU C++ does not support old-style function definitions, so this
|
|
extension is irrelevant.
|
|
|
|
@node C++ Comments
|
|
@section C++ Style Comments
|
|
@cindex //
|
|
@cindex C++ comments
|
|
@cindex comments, C++ style
|
|
|
|
In GNU C, you may use C++ style comments, which start with @samp{//} and
|
|
continue until the end of the line. Many other C implementations allow
|
|
such comments, and they are likely to be in a future C standard.
|
|
However, C++ style comments are not recognized if you specify
|
|
@w{@samp{-ansi}} or @w{@samp{-traditional}}, since they are incompatible
|
|
with traditional constructs like @code{dividend//*comment*/divisor}.
|
|
|
|
@node Dollar Signs
|
|
@section Dollar Signs in Identifier Names
|
|
@cindex $
|
|
@cindex dollar signs in identifier names
|
|
@cindex identifier names, dollar signs in
|
|
|
|
In GNU C, you may normally use dollar signs in identifier names.
|
|
This is because many traditional C implementations allow such identifiers.
|
|
However, dollar signs in identifiers are not supported on a few target
|
|
machines, typically because the target assembler does not allow them.
|
|
|
|
@node Character Escapes
|
|
@section The Character @key{ESC} in Constants
|
|
|
|
You can use the sequence @samp{\e} in a string or character constant to
|
|
stand for the ASCII character @key{ESC}.
|
|
|
|
@node Alignment
|
|
@section Inquiring on Alignment of Types or Variables
|
|
@cindex alignment
|
|
@cindex type alignment
|
|
@cindex variable alignment
|
|
|
|
The keyword @code{__alignof__} allows you to inquire about how an object
|
|
is aligned, or the minimum alignment usually required by a type. Its
|
|
syntax is just like @code{sizeof}.
|
|
|
|
For example, if the target machine requires a @code{double} value to be
|
|
aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8.
|
|
This is true on many RISC machines. On more traditional machine
|
|
designs, @code{__alignof__ (double)} is 4 or even 2.
|
|
|
|
Some machines never actually require alignment; they allow reference to any
|
|
data type even at an odd addresses. For these machines, @code{__alignof__}
|
|
reports the @emph{recommended} alignment of a type.
|
|
|
|
When the operand of @code{__alignof__} is an lvalue rather than a type, the
|
|
value is the largest alignment that the lvalue is known to have. It may
|
|
have this alignment as a result of its data type, or because it is part of
|
|
a structure and inherits alignment from that structure. For example, after
|
|
this declaration:
|
|
|
|
@example
|
|
struct foo @{ int x; char y; @} foo1;
|
|
@end example
|
|
|
|
@noindent
|
|
the value of @code{__alignof__ (foo1.y)} is probably 2 or 4, the same as
|
|
@code{__alignof__ (int)}, even though the data type of @code{foo1.y}
|
|
does not itself demand any alignment.@refill
|
|
|
|
A related feature which lets you specify the alignment of an object is
|
|
@code{__attribute__ ((aligned (@var{alignment})))}; see the following
|
|
section.
|
|
|
|
@node Variable Attributes
|
|
@section Specifying Attributes of Variables
|
|
@cindex attribute of variables
|
|
@cindex variable attributes
|
|
|
|
The keyword @code{__attribute__} allows you to specify special
|
|
attributes of variables or structure fields. This keyword is followed
|
|
by an attribute specification inside double parentheses. Eight
|
|
attributes are currently defined for variables: @code{aligned},
|
|
@code{mode}, @code{nocommon}, @code{packed}, @code{section},
|
|
@code{transparent_union}, @code{unused}, and @code{weak}. Other
|
|
attributes are available for functions (@pxref{Function Attributes}) and
|
|
for types (@pxref{Type Attributes}).
|
|
|
|
You may also specify attributes with @samp{__} preceding and following
|
|
each keyword. This allows you to use them in header files without
|
|
being concerned about a possible macro of the same name. For example,
|
|
you may use @code{__aligned__} instead of @code{aligned}.
|
|
|
|
@table @code
|
|
@cindex @code{aligned} attribute
|
|
@item aligned (@var{alignment})
|
|
This attribute specifies a minimum alignment for the variable or
|
|
structure field, measured in bytes. For example, the declaration:
|
|
|
|
@smallexample
|
|
int x __attribute__ ((aligned (16))) = 0;
|
|
@end smallexample
|
|
|
|
@noindent
|
|
causes the compiler to allocate the global variable @code{x} on a
|
|
16-byte boundary. On a 68040, this could be used in conjunction with
|
|
an @code{asm} expression to access the @code{move16} instruction which
|
|
requires 16-byte aligned operands.
|
|
|
|
You can also specify the alignment of structure fields. For example, to
|
|
create a double-word aligned @code{int} pair, you could write:
|
|
|
|
@smallexample
|
|
struct foo @{ int x[2] __attribute__ ((aligned (8))); @};
|
|
@end smallexample
|
|
|
|
@noindent
|
|
This is an alternative to creating a union with a @code{double} member
|
|
that forces the union to be double-word aligned.
|
|
|
|
It is not possible to specify the alignment of functions; the alignment
|
|
of functions is determined by the machine's requirements and cannot be
|
|
changed. You cannot specify alignment for a typedef name because such a
|
|
name is just an alias, not a distinct type.
|
|
|
|
As in the preceding examples, you can explicitly specify the alignment
|
|
(in bytes) that you wish the compiler to use for a given variable or
|
|
structure field. Alternatively, you can leave out the alignment factor
|
|
and just ask the compiler to align a variable or field to the maximum
|
|
useful alignment for the target machine you are compiling for. For
|
|
example, you could write:
|
|
|
|
@smallexample
|
|
short array[3] __attribute__ ((aligned));
|
|
@end smallexample
|
|
|
|
Whenever you leave out the alignment factor in an @code{aligned} attribute
|
|
specification, the compiler automatically sets the alignment for the declared
|
|
variable or field to the largest alignment which is ever used for any data
|
|
type on the target machine you are compiling for. Doing this can often make
|
|
copy operations more efficient, because the compiler can use whatever
|
|
instructions copy the biggest chunks of memory when performing copies to
|
|
or from the variables or fields that you have aligned this way.
|
|
|
|
The @code{aligned} attribute can only increase the alignment; but you
|
|
can decrease it by specifying @code{packed} as well. See below.
|
|
|
|
Note that the effectiveness of @code{aligned} attributes may be limited
|
|
by inherent limitations in your linker. On many systems, the linker is
|
|
only able to arrange for variables to be aligned up to a certain maximum
|
|
alignment. (For some linkers, the maximum supported alignment may
|
|
be very very small.) If your linker is only able to align variables
|
|
up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
|
|
in an @code{__attribute__} will still only provide you with 8 byte
|
|
alignment. See your linker documentation for further information.
|
|
|
|
@item mode (@var{mode})
|
|
@cindex @code{mode} attribute
|
|
This attribute specifies the data type for the declaration---whichever
|
|
type corresponds to the mode @var{mode}. This in effect lets you
|
|
request an integer or floating point type according to its width.
|
|
|
|
You may also specify a mode of @samp{byte} or @samp{__byte__} to
|
|
indicate the mode corresponding to a one-byte integer, @samp{word} or
|
|
@samp{__word__} for the mode of a one-word integer, and @samp{pointer}
|
|
or @samp{__pointer__} for the mode used to represent pointers.
|
|
|
|
@item nocommon
|
|
@cindex @code{nocommon} attribute
|
|
This attribute specifies requests GNU CC not to place a variable
|
|
``common'' but instead to allocate space for it directly. If you
|
|
specify the @samp{-fno-common} flag, GNU CC will do this for all
|
|
variables.
|
|
|
|
Specifying the @code{nocommon} attribute for a variable provides an
|
|
initialization of zeros. A variable may only be initialized in one
|
|
source file.
|
|
|
|
@item packed
|
|
@cindex @code{packed} attribute
|
|
The @code{packed} attribute specifies that a variable or structure field
|
|
should have the smallest possible alignment---one byte for a variable,
|
|
and one bit for a field, unless you specify a larger value with the
|
|
@code{aligned} attribute.
|
|
|
|
Here is a structure in which the field @code{x} is packed, so that it
|
|
immediately follows @code{a}:
|
|
|
|
@example
|
|
struct foo
|
|
@{
|
|
char a;
|
|
int x[2] __attribute__ ((packed));
|
|
@};
|
|
@end example
|
|
|
|
@item section ("section-name")
|
|
@cindex @code{section} variable attribute
|
|
Normally, the compiler places the objects it generates in sections like
|
|
@code{data} and @code{bss}. Sometimes, however, you need additional sections,
|
|
or you need certain particular variables to appear in special sections,
|
|
for example to map to special hardware. The @code{section}
|
|
attribute specifies that a variable (or function) lives in a particular
|
|
section. For example, this small program uses several specific section names:
|
|
|
|
@smallexample
|
|
struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @};
|
|
struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @};
|
|
char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @};
|
|
int init_data __attribute__ ((section ("INITDATA"))) = 0;
|
|
|
|
main()
|
|
@{
|
|
/* Initialize stack pointer */
|
|
init_sp (stack + sizeof (stack));
|
|
|
|
/* Initialize initialized data */
|
|
memcpy (&init_data, &data, &edata - &data);
|
|
|
|
/* Turn on the serial ports */
|
|
init_duart (&a);
|
|
init_duart (&b);
|
|
@}
|
|
@end smallexample
|
|
|
|
@noindent
|
|
Use the @code{section} attribute with an @emph{initialized} definition
|
|
of a @emph{global} variable, as shown in the example. GNU CC issues
|
|
a warning and otherwise ignores the @code{section} attribute in
|
|
uninitialized variable declarations.
|
|
|
|
You may only use the @code{section} attribute with a fully initialized
|
|
global definition because of the way linkers work. The linker requires
|
|
each object be defined once, with the exception that uninitialized
|
|
variables tentatively go in the @code{common} (or @code{bss}) section
|
|
and can be multiply "defined". You can force a variable to be
|
|
initialized with the @samp{-fno-common} flag or the @code{nocommon}
|
|
attribute.
|
|
|
|
Some file formats do not support arbitrary sections so the @code{section}
|
|
attribute is not available on all platforms.
|
|
If you need to map the entire contents of a module to a particular
|
|
section, consider using the facilities of the linker instead.
|
|
|
|
@item transparent_union
|
|
This attribute, attached to a function parameter which is a union, means
|
|
that the corresponding argument may have the type of any union member,
|
|
but the argument is passed as if its type were that of the first union
|
|
member. For more details see @xref{Type Attributes}. You can also use
|
|
this attribute on a @code{typedef} for a union data type; then it
|
|
applies to all function parameters with that type.
|
|
|
|
@item unused
|
|
This attribute, attached to a variable, means that the variable is meant
|
|
to be possibly unused. GNU CC will not produce a warning for this
|
|
variable.
|
|
|
|
@item weak
|
|
The @code{weak} attribute is described in @xref{Function Attributes}.
|
|
|
|
@item model (@var{model-name})
|
|
@cindex variable addressability on the M32R/D
|
|
Use this attribute on the M32R/D to set the addressability of an object.
|
|
The identifier @var{model-name} is one of @code{small}, @code{medium},
|
|
or @code{large}, representing each of the code models.
|
|
|
|
Small model objects live in the lower 16MB of memory (so that their
|
|
addresses can be loaded with the @code{ld24} instruction).
|
|
|
|
Medium and large model objects may live anywhere in the 32 bit address space
|
|
(the compiler will generate @code{seth/add3} instructions to load their
|
|
addresses).
|
|
|
|
@end table
|
|
|
|
To specify multiple attributes, separate them by commas within the
|
|
double parentheses: for example, @samp{__attribute__ ((aligned (16),
|
|
packed))}.
|
|
|
|
@node Type Attributes
|
|
@section Specifying Attributes of Types
|
|
@cindex attribute of types
|
|
@cindex type attributes
|
|
|
|
The keyword @code{__attribute__} allows you to specify special
|
|
attributes of @code{struct} and @code{union} types when you define such
|
|
types. This keyword is followed by an attribute specification inside
|
|
double parentheses. Three attributes are currently defined for types:
|
|
@code{aligned}, @code{packed}, and @code{transparent_union}. Other
|
|
attributes are defined for functions (@pxref{Function Attributes}) and
|
|
for variables (@pxref{Variable Attributes}).
|
|
|
|
You may also specify any one of these attributes with @samp{__}
|
|
preceding and following its keyword. This allows you to use these
|
|
attributes in header files without being concerned about a possible
|
|
macro of the same name. For example, you may use @code{__aligned__}
|
|
instead of @code{aligned}.
|
|
|
|
You may specify the @code{aligned} and @code{transparent_union}
|
|
attributes either in a @code{typedef} declaration or just past the
|
|
closing curly brace of a complete enum, struct or union type
|
|
@emph{definition} and the @code{packed} attribute only past the closing
|
|
brace of a definition.
|
|
|
|
You may also specify attributes between the enum, struct or union
|
|
tag and the name of the type rather than after the closing brace.
|
|
|
|
@table @code
|
|
@cindex @code{aligned} attribute
|
|
@item aligned (@var{alignment})
|
|
This attribute specifies a minimum alignment (in bytes) for variables
|
|
of the specified type. For example, the declarations:
|
|
|
|
@smallexample
|
|
struct S @{ short f[3]; @} __attribute__ ((aligned (8)));
|
|
typedef int more_aligned_int __attribute__ ((aligned (8)));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
force the compiler to insure (as far as it can) that each variable whose
|
|
type is @code{struct S} or @code{more_aligned_int} will be allocated and
|
|
aligned @emph{at least} on a 8-byte boundary. On a Sparc, having all
|
|
variables of type @code{struct S} aligned to 8-byte boundaries allows
|
|
the compiler to use the @code{ldd} and @code{std} (doubleword load and
|
|
store) instructions when copying one variable of type @code{struct S} to
|
|
another, thus improving run-time efficiency.
|
|
|
|
Note that the alignment of any given @code{struct} or @code{union} type
|
|
is required by the ANSI C standard to be at least a perfect multiple of
|
|
the lowest common multiple of the alignments of all of the members of
|
|
the @code{struct} or @code{union} in question. This means that you @emph{can}
|
|
effectively adjust the alignment of a @code{struct} or @code{union}
|
|
type by attaching an @code{aligned} attribute to any one of the members
|
|
of such a type, but the notation illustrated in the example above is a
|
|
more obvious, intuitive, and readable way to request the compiler to
|
|
adjust the alignment of an entire @code{struct} or @code{union} type.
|
|
|
|
As in the preceding example, you can explicitly specify the alignment
|
|
(in bytes) that you wish the compiler to use for a given @code{struct}
|
|
or @code{union} type. Alternatively, you can leave out the alignment factor
|
|
and just ask the compiler to align a type to the maximum
|
|
useful alignment for the target machine you are compiling for. For
|
|
example, you could write:
|
|
|
|
@smallexample
|
|
struct S @{ short f[3]; @} __attribute__ ((aligned));
|
|
@end smallexample
|
|
|
|
Whenever you leave out the alignment factor in an @code{aligned}
|
|
attribute specification, the compiler automatically sets the alignment
|
|
for the type to the largest alignment which is ever used for any data
|
|
type on the target machine you are compiling for. Doing this can often
|
|
make copy operations more efficient, because the compiler can use
|
|
whatever instructions copy the biggest chunks of memory when performing
|
|
copies to or from the variables which have types that you have aligned
|
|
this way.
|
|
|
|
In the example above, if the size of each @code{short} is 2 bytes, then
|
|
the size of the entire @code{struct S} type is 6 bytes. The smallest
|
|
power of two which is greater than or equal to that is 8, so the
|
|
compiler sets the alignment for the entire @code{struct S} type to 8
|
|
bytes.
|
|
|
|
Note that although you can ask the compiler to select a time-efficient
|
|
alignment for a given type and then declare only individual stand-alone
|
|
objects of that type, the compiler's ability to select a time-efficient
|
|
alignment is primarily useful only when you plan to create arrays of
|
|
variables having the relevant (efficiently aligned) type. If you
|
|
declare or use arrays of variables of an efficiently-aligned type, then
|
|
it is likely that your program will also be doing pointer arithmetic (or
|
|
subscripting, which amounts to the same thing) on pointers to the
|
|
relevant type, and the code that the compiler generates for these
|
|
pointer arithmetic operations will often be more efficient for
|
|
efficiently-aligned types than for other types.
|
|
|
|
The @code{aligned} attribute can only increase the alignment; but you
|
|
can decrease it by specifying @code{packed} as well. See below.
|
|
|
|
Note that the effectiveness of @code{aligned} attributes may be limited
|
|
by inherent limitations in your linker. On many systems, the linker is
|
|
only able to arrange for variables to be aligned up to a certain maximum
|
|
alignment. (For some linkers, the maximum supported alignment may
|
|
be very very small.) If your linker is only able to align variables
|
|
up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
|
|
in an @code{__attribute__} will still only provide you with 8 byte
|
|
alignment. See your linker documentation for further information.
|
|
|
|
@item packed
|
|
This attribute, attached to an @code{enum}, @code{struct}, or
|
|
@code{union} type definition, specified that the minimum required memory
|
|
be used to represent the type.
|
|
|
|
Specifying this attribute for @code{struct} and @code{union} types is
|
|
equivalent to specifying the @code{packed} attribute on each of the
|
|
structure or union members. Specifying the @samp{-fshort-enums}
|
|
flag on the line is equivalent to specifying the @code{packed}
|
|
attribute on all @code{enum} definitions.
|
|
|
|
You may only specify this attribute after a closing curly brace on an
|
|
@code{enum} definition, not in a @code{typedef} declaration, unless that
|
|
declaration also contains the definition of the @code{enum}.
|
|
|
|
@item transparent_union
|
|
This attribute, attached to a @code{union} type definition, indicates
|
|
that any function parameter having that union type causes calls to that
|
|
function to be treated in a special way.
|
|
|
|
First, the argument corresponding to a transparent union type can be of
|
|
any type in the union; no cast is required. Also, if the union contains
|
|
a pointer type, the corresponding argument can be a null pointer
|
|
constant or a void pointer expression; and if the union contains a void
|
|
pointer type, the corresponding argument can be any pointer expression.
|
|
If the union member type is a pointer, qualifiers like @code{const} on
|
|
the referenced type must be respected, just as with normal pointer
|
|
conversions.
|
|
|
|
Second, the argument is passed to the function using the calling
|
|
conventions of first member of the transparent union, not the calling
|
|
conventions of the union itself. All members of the union must have the
|
|
same machine representation; this is necessary for this argument passing
|
|
to work properly.
|
|
|
|
Transparent unions are designed for library functions that have multiple
|
|
interfaces for compatibility reasons. For example, suppose the
|
|
@code{wait} function must accept either a value of type @code{int *} to
|
|
comply with Posix, or a value of type @code{union wait *} to comply with
|
|
the 4.1BSD interface. If @code{wait}'s parameter were @code{void *},
|
|
@code{wait} would accept both kinds of arguments, but it would also
|
|
accept any other pointer type and this would make argument type checking
|
|
less useful. Instead, @code{<sys/wait.h>} might define the interface
|
|
as follows:
|
|
|
|
@smallexample
|
|
typedef union
|
|
@{
|
|
int *__ip;
|
|
union wait *__up;
|
|
@} wait_status_ptr_t __attribute__ ((__transparent_union__));
|
|
|
|
pid_t wait (wait_status_ptr_t);
|
|
@end smallexample
|
|
|
|
This interface allows either @code{int *} or @code{union wait *}
|
|
arguments to be passed, using the @code{int *} calling convention.
|
|
The program can call @code{wait} with arguments of either type:
|
|
|
|
@example
|
|
int w1 () @{ int w; return wait (&w); @}
|
|
int w2 () @{ union wait w; return wait (&w); @}
|
|
@end example
|
|
|
|
With this interface, @code{wait}'s implementation might look like this:
|
|
|
|
@example
|
|
pid_t wait (wait_status_ptr_t p)
|
|
@{
|
|
return waitpid (-1, p.__ip, 0);
|
|
@}
|
|
@end example
|
|
|
|
@item unused
|
|
When attached to a type (including a @code{union} or a @code{struct}),
|
|
this attribute means that variables of that type are meant to appear
|
|
possibly unused. GNU CC will not produce a warning for any variables of
|
|
that type, even if the variable appears to do nothing. This is often
|
|
the case with lock or thread classes, which are usually defined and then
|
|
not referenced, but contain constructors and destructors that have
|
|
nontrivial bookkeeping functions.
|
|
|
|
@end table
|
|
|
|
To specify multiple attributes, separate them by commas within the
|
|
double parentheses: for example, @samp{__attribute__ ((aligned (16),
|
|
packed))}.
|
|
|
|
@node Inline
|
|
@section An Inline Function is As Fast As a Macro
|
|
@cindex inline functions
|
|
@cindex integrating function code
|
|
@cindex open coding
|
|
@cindex macros, inline alternative
|
|
|
|
By declaring a function @code{inline}, you can direct GNU CC to
|
|
integrate that function's code into the code for its callers. This
|
|
makes execution faster by eliminating the function-call overhead; in
|
|
addition, if any of the actual argument values are constant, their known
|
|
values may permit simplifications at compile time so that not all of the
|
|
inline function's code needs to be included. The effect on code size is
|
|
less predictable; object code may be larger or smaller with function
|
|
inlining, depending on the particular case. Inlining of functions is an
|
|
optimization and it really ``works'' only in optimizing compilation. If
|
|
you don't use @samp{-O}, no function is really inline.
|
|
|
|
To declare a function inline, use the @code{inline} keyword in its
|
|
declaration, like this:
|
|
|
|
@example
|
|
inline int
|
|
inc (int *a)
|
|
@{
|
|
(*a)++;
|
|
@}
|
|
@end example
|
|
|
|
(If you are writing a header file to be included in ANSI C programs, write
|
|
@code{__inline__} instead of @code{inline}. @xref{Alternate Keywords}.)
|
|
|
|
You can also make all ``simple enough'' functions inline with the option
|
|
@samp{-finline-functions}. Note that certain usages in a function
|
|
definition can make it unsuitable for inline substitution.
|
|
|
|
Note that in C and Objective C, unlike C++, the @code{inline} keyword
|
|
does not affect the linkage of the function.
|
|
|
|
@cindex automatic @code{inline} for C++ member fns
|
|
@cindex @code{inline} automatic for C++ member fns
|
|
@cindex member fns, automatically @code{inline}
|
|
@cindex C++ member fns, automatically @code{inline}
|
|
GNU CC automatically inlines member functions defined within the class
|
|
body of C++ programs even if they are not explicitly declared
|
|
@code{inline}. (You can override this with @samp{-fno-default-inline};
|
|
@pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.)
|
|
|
|
@cindex inline functions, omission of
|
|
When a function is both inline and @code{static}, if all calls to the
|
|
function are integrated into the caller, and the function's address is
|
|
never used, then the function's own assembler code is never referenced.
|
|
In this case, GNU CC does not actually output assembler code for the
|
|
function, unless you specify the option @samp{-fkeep-inline-functions}.
|
|
Some calls cannot be integrated for various reasons (in particular,
|
|
calls that precede the function's definition cannot be integrated, and
|
|
neither can recursive calls within the definition). If there is a
|
|
nonintegrated call, then the function is compiled to assembler code as
|
|
usual. The function must also be compiled as usual if the program
|
|
refers to its address, because that can't be inlined.
|
|
|
|
@cindex non-static inline function
|
|
When an inline function is not @code{static}, then the compiler must assume
|
|
that there may be calls from other source files; since a global symbol can
|
|
be defined only once in any program, the function must not be defined in
|
|
the other source files, so the calls therein cannot be integrated.
|
|
Therefore, a non-@code{static} inline function is always compiled on its
|
|
own in the usual fashion.
|
|
|
|
If you specify both @code{inline} and @code{extern} in the function
|
|
definition, then the definition is used only for inlining. In no case
|
|
is the function compiled on its own, not even if you refer to its
|
|
address explicitly. Such an address becomes an external reference, as
|
|
if you had only declared the function, and had not defined it.
|
|
|
|
This combination of @code{inline} and @code{extern} has almost the
|
|
effect of a macro. The way to use it is to put a function definition in
|
|
a header file with these keywords, and put another copy of the
|
|
definition (lacking @code{inline} and @code{extern}) in a library file.
|
|
The definition in the header file will cause most calls to the function
|
|
to be inlined. If any uses of the function remain, they will refer to
|
|
the single copy in the library.
|
|
|
|
GNU C does not inline any functions when not optimizing. It is not
|
|
clear whether it is better to inline or not, in this case, but we found
|
|
that a correct implementation when not optimizing was difficult. So we
|
|
did the easy thing, and turned it off.
|
|
|
|
@node Extended Asm
|
|
@section Assembler Instructions with C Expression Operands
|
|
@cindex extended @code{asm}
|
|
@cindex @code{asm} expressions
|
|
@cindex assembler instructions
|
|
@cindex registers
|
|
|
|
In an assembler instruction using @code{asm}, you can specify the
|
|
operands of the instruction using C expressions. This means you need not
|
|
guess which registers or memory locations will contain the data you want
|
|
to use.
|
|
|
|
You must specify an assembler instruction template much like what
|
|
appears in a machine description, plus an operand constraint string for
|
|
each operand.
|
|
|
|
For example, here is how to use the 68881's @code{fsinx} instruction:
|
|
|
|
@example
|
|
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
|
|
@end example
|
|
|
|
@noindent
|
|
Here @code{angle} is the C expression for the input operand while
|
|
@code{result} is that of the output operand. Each has @samp{"f"} as its
|
|
operand constraint, saying that a floating point register is required.
|
|
The @samp{=} in @samp{=f} indicates that the operand is an output; all
|
|
output operands' constraints must use @samp{=}. The constraints use the
|
|
same language used in the machine description (@pxref{Constraints}).
|
|
|
|
Each operand is described by an operand-constraint string followed by
|
|
the C expression in parentheses. A colon separates the assembler
|
|
template from the first output operand and another separates the last
|
|
output operand from the first input, if any. Commas separate the
|
|
operands within each group. The total number of operands is limited to
|
|
ten or to the maximum number of operands in any instruction pattern in
|
|
the machine description, whichever is greater.
|
|
|
|
If there are no output operands but there are input operands, you must
|
|
place two consecutive colons surrounding the place where the output
|
|
operands would go.
|
|
|
|
Output operand expressions must be lvalues; the compiler can check this.
|
|
The input operands need not be lvalues. The compiler cannot check
|
|
whether the operands have data types that are reasonable for the
|
|
instruction being executed. It does not parse the assembler instruction
|
|
template and does not know what it means or even whether it is valid
|
|
assembler input. The extended @code{asm} feature is most often used for
|
|
machine instructions the compiler itself does not know exist. If
|
|
the output expression cannot be directly addressed (for example, it is a
|
|
bit field), your constraint must allow a register. In that case, GNU CC
|
|
will use the register as the output of the @code{asm}, and then store
|
|
that register into the output.
|
|
|
|
The ordinary output operands must be write-only; GNU CC will assume that
|
|
the values in these operands before the instruction are dead and need
|
|
not be generated. Extended asm supports input-output or read-write
|
|
operands. Use the constraint character @samp{+} to indicate such an
|
|
operand and list it with the output operands.
|
|
|
|
When the constraints for the read-write operand (or the operand in which
|
|
only some of the bits are to be changed) allows a register, you may, as
|
|
an alternative, logically split its function into two separate operands,
|
|
one input operand and one write-only output operand. The connection
|
|
between them is expressed by constraints which say they need to be in
|
|
the same location when the instruction executes. You can use the same C
|
|
expression for both operands, or different expressions. For example,
|
|
here we write the (fictitious) @samp{combine} instruction with
|
|
@code{bar} as its read-only source operand and @code{foo} as its
|
|
read-write destination:
|
|
|
|
@example
|
|
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
|
|
@end example
|
|
|
|
@noindent
|
|
The constraint @samp{"0"} for operand 1 says that it must occupy the
|
|
same location as operand 0. A digit in constraint is allowed only in an
|
|
input operand and it must refer to an output operand.
|
|
|
|
Only a digit in the constraint can guarantee that one operand will be in
|
|
the same place as another. The mere fact that @code{foo} is the value
|
|
of both operands is not enough to guarantee that they will be in the
|
|
same place in the generated assembler code. The following would not
|
|
work reliably:
|
|
|
|
@example
|
|
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
|
|
@end example
|
|
|
|
Various optimizations or reloading could cause operands 0 and 1 to be in
|
|
different registers; GNU CC knows no reason not to do so. For example, the
|
|
compiler might find a copy of the value of @code{foo} in one register and
|
|
use it for operand 1, but generate the output operand 0 in a different
|
|
register (copying it afterward to @code{foo}'s own address). Of course,
|
|
since the register for operand 1 is not even mentioned in the assembler
|
|
code, the result will not work, but GNU CC can't tell that.
|
|
|
|
Some instructions clobber specific hard registers. To describe this,
|
|
write a third colon after the input operands, followed by the names of
|
|
the clobbered hard registers (given as strings). Here is a realistic
|
|
example for the VAX:
|
|
|
|
@example
|
|
asm volatile ("movc3 %0,%1,%2"
|
|
: /* no outputs */
|
|
: "g" (from), "g" (to), "g" (count)
|
|
: "r0", "r1", "r2", "r3", "r4", "r5");
|
|
@end example
|
|
|
|
If you refer to a particular hardware register from the assembler code,
|
|
you will probably have to list the register after the third colon to
|
|
tell the compiler the register's value is modified. In some assemblers,
|
|
the register names begin with @samp{%}; to produce one @samp{%} in the
|
|
assembler code, you must write @samp{%%} in the input.
|
|
|
|
If your assembler instruction can alter the condition code register, add
|
|
@samp{cc} to the list of clobbered registers. GNU CC on some machines
|
|
represents the condition codes as a specific hardware register;
|
|
@samp{cc} serves to name this register. On other machines, the
|
|
condition code is handled differently, and specifying @samp{cc} has no
|
|
effect. But it is valid no matter what the machine.
|
|
|
|
If your assembler instruction modifies memory in an unpredictable
|
|
fashion, add @samp{memory} to the list of clobbered registers. This
|
|
will cause GNU CC to not keep memory values cached in registers across
|
|
the assembler instruction.
|
|
|
|
You can put multiple assembler instructions together in a single
|
|
@code{asm} template, separated either with newlines (written as
|
|
@samp{\n}) or with semicolons if the assembler allows such semicolons.
|
|
The GNU assembler allows semicolons and most Unix assemblers seem to do
|
|
so. The input operands are guaranteed not to use any of the clobbered
|
|
registers, and neither will the output operands' addresses, so you can
|
|
read and write the clobbered registers as many times as you like. Here
|
|
is an example of multiple instructions in a template; it assumes the
|
|
subroutine @code{_foo} accepts arguments in registers 9 and 10:
|
|
|
|
@example
|
|
asm ("movl %0,r9;movl %1,r10;call _foo"
|
|
: /* no outputs */
|
|
: "g" (from), "g" (to)
|
|
: "r9", "r10");
|
|
@end example
|
|
|
|
Unless an output operand has the @samp{&} constraint modifier, GNU CC
|
|
may allocate it in the same register as an unrelated input operand, on
|
|
the assumption the inputs are consumed before the outputs are produced.
|
|
This assumption may be false if the assembler code actually consists of
|
|
more than one instruction. In such a case, use @samp{&} for each output
|
|
operand that may not overlap an input. @xref{Modifiers}.
|
|
|
|
If you want to test the condition code produced by an assembler
|
|
instruction, you must include a branch and a label in the @code{asm}
|
|
construct, as follows:
|
|
|
|
@example
|
|
asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
|
|
: "g" (result)
|
|
: "g" (input));
|
|
@end example
|
|
|
|
@noindent
|
|
This assumes your assembler supports local labels, as the GNU assembler
|
|
and most Unix assemblers do.
|
|
|
|
Speaking of labels, jumps from one @code{asm} to another are not
|
|
supported. The compiler's optimizers do not know about these jumps, and
|
|
therefore they cannot take account of them when deciding how to
|
|
optimize.
|
|
|
|
@cindex macros containing @code{asm}
|
|
Usually the most convenient way to use these @code{asm} instructions is to
|
|
encapsulate them in macros that look like functions. For example,
|
|
|
|
@example
|
|
#define sin(x) \
|
|
(@{ double __value, __arg = (x); \
|
|
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
|
|
__value; @})
|
|
@end example
|
|
|
|
@noindent
|
|
Here the variable @code{__arg} is used to make sure that the instruction
|
|
operates on a proper @code{double} value, and to accept only those
|
|
arguments @code{x} which can convert automatically to a @code{double}.
|
|
|
|
Another way to make sure the instruction operates on the correct data
|
|
type is to use a cast in the @code{asm}. This is different from using a
|
|
variable @code{__arg} in that it converts more different types. For
|
|
example, if the desired type were @code{int}, casting the argument to
|
|
@code{int} would accept a pointer with no complaint, while assigning the
|
|
argument to an @code{int} variable named @code{__arg} would warn about
|
|
using a pointer unless the caller explicitly casts it.
|
|
|
|
If an @code{asm} has output operands, GNU CC assumes for optimization
|
|
purposes the instruction has no side effects except to change the output
|
|
operands. This does not mean instructions with a side effect cannot be
|
|
used, but you must be careful, because the compiler may eliminate them
|
|
if the output operands aren't used, or move them out of loops, or
|
|
replace two with one if they constitute a common subexpression. Also,
|
|
if your instruction does have a side effect on a variable that otherwise
|
|
appears not to change, the old value of the variable may be reused later
|
|
if it happens to be found in a register.
|
|
|
|
You can prevent an @code{asm} instruction from being deleted, moved
|
|
significantly, or combined, by writing the keyword @code{volatile} after
|
|
the @code{asm}. For example:
|
|
|
|
@example
|
|
#define get_and_set_priority(new) \
|
|
(@{ int __old; \
|
|
asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \
|
|
__old; @})
|
|
b@end example
|
|
|
|
@noindent
|
|
If you write an @code{asm} instruction with no outputs, GNU CC will know
|
|
the instruction has side-effects and will not delete the instruction or
|
|
move it outside of loops. If the side-effects of your instruction are
|
|
not purely external, but will affect variables in your program in ways
|
|
other than reading the inputs and clobbering the specified registers or
|
|
memory, you should write the @code{volatile} keyword to prevent future
|
|
versions of GNU CC from moving the instruction around within a core
|
|
region.
|
|
|
|
An @code{asm} instruction without any operands or clobbers (and ``old
|
|
style'' @code{asm}) will not be deleted or moved significantly,
|
|
regardless, unless it is unreachable, the same wasy as if you had
|
|
written a @code{volatile} keyword.
|
|
|
|
Note that even a volatile @code{asm} instruction can be moved in ways
|
|
that appear insignificant to the compiler, such as across jump
|
|
instructions. You can't expect a sequence of volatile @code{asm}
|
|
instructions to remain perfectly consecutive. If you want consecutive
|
|
output, use a single @code{asm}.
|
|
|
|
It is a natural idea to look for a way to give access to the condition
|
|
code left by the assembler instruction. However, when we attempted to
|
|
implement this, we found no way to make it work reliably. The problem
|
|
is that output operands might need reloading, which would result in
|
|
additional following ``store'' instructions. On most machines, these
|
|
instructions would alter the condition code before there was time to
|
|
test it. This problem doesn't arise for ordinary ``test'' and
|
|
``compare'' instructions because they don't have any output operands.
|
|
|
|
If you are writing a header file that should be includable in ANSI C
|
|
programs, write @code{__asm__} instead of @code{asm}. @xref{Alternate
|
|
Keywords}.
|
|
|
|
@ifclear INTERNALS
|
|
@c Show the details on constraints if they do not appear elsewhere in
|
|
@c the manual
|
|
@include md.texi
|
|
@end ifclear
|
|
|
|
@node Asm Labels
|
|
@section Controlling Names Used in Assembler Code
|
|
@cindex assembler names for identifiers
|
|
@cindex names used in assembler code
|
|
@cindex identifiers, names in assembler code
|
|
|
|
You can specify the name to be used in the assembler code for a C
|
|
function or variable by writing the @code{asm} (or @code{__asm__})
|
|
keyword after the declarator as follows:
|
|
|
|
@example
|
|
int foo asm ("myfoo") = 2;
|
|
@end example
|
|
|
|
@noindent
|
|
This specifies that the name to be used for the variable @code{foo} in
|
|
the assembler code should be @samp{myfoo} rather than the usual
|
|
@samp{_foo}.
|
|
|
|
On systems where an underscore is normally prepended to the name of a C
|
|
function or variable, this feature allows you to define names for the
|
|
linker that do not start with an underscore.
|
|
|
|
You cannot use @code{asm} in this way in a function @emph{definition}; but
|
|
you can get the same effect by writing a declaration for the function
|
|
before its definition and putting @code{asm} there, like this:
|
|
|
|
@example
|
|
extern func () asm ("FUNC");
|
|
|
|
func (x, y)
|
|
int x, y;
|
|
@dots{}
|
|
@end example
|
|
|
|
It is up to you to make sure that the assembler names you choose do not
|
|
conflict with any other assembler symbols. Also, you must not use a
|
|
register name; that would produce completely invalid assembler code. GNU
|
|
CC does not as yet have the ability to store static variables in registers.
|
|
Perhaps that will be added.
|
|
|
|
@node Explicit Reg Vars
|
|
@section Variables in Specified Registers
|
|
@cindex explicit register variables
|
|
@cindex variables in specified registers
|
|
@cindex specified registers
|
|
@cindex registers, global allocation
|
|
|
|
GNU C allows you to put a few global variables into specified hardware
|
|
registers. You can also specify the register in which an ordinary
|
|
register variable should be allocated.
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Global register variables reserve registers throughout the program.
|
|
This may be useful in programs such as programming language
|
|
interpreters which have a couple of global variables that are accessed
|
|
very often.
|
|
|
|
@item
|
|
Local register variables in specific registers do not reserve the
|
|
registers. The compiler's data flow analysis is capable of determining
|
|
where the specified registers contain live values, and where they are
|
|
available for other uses.
|
|
|
|
These local variables are sometimes convenient for use with the extended
|
|
@code{asm} feature (@pxref{Extended Asm}), if you want to write one
|
|
output of the assembler instruction directly into a particular register.
|
|
(This will work provided the register you specify fits the constraints
|
|
specified for that operand in the @code{asm}.)
|
|
@end itemize
|
|
|
|
@menu
|
|
* Global Reg Vars::
|
|
* Local Reg Vars::
|
|
@end menu
|
|
|
|
@node Global Reg Vars
|
|
@subsection Defining Global Register Variables
|
|
@cindex global register variables
|
|
@cindex registers, global variables in
|
|
|
|
You can define a global register variable in GNU C like this:
|
|
|
|
@example
|
|
register int *foo asm ("a5");
|
|
@end example
|
|
|
|
@noindent
|
|
Here @code{a5} is the name of the register which should be used. Choose a
|
|
register which is normally saved and restored by function calls on your
|
|
machine, so that library routines will not clobber it.
|
|
|
|
Naturally the register name is cpu-dependent, so you would need to
|
|
conditionalize your program according to cpu type. The register
|
|
@code{a5} would be a good choice on a 68000 for a variable of pointer
|
|
type. On machines with register windows, be sure to choose a ``global''
|
|
register that is not affected magically by the function call mechanism.
|
|
|
|
In addition, operating systems on one type of cpu may differ in how they
|
|
name the registers; then you would need additional conditionals. For
|
|
example, some 68000 operating systems call this register @code{%a5}.
|
|
|
|
Eventually there may be a way of asking the compiler to choose a register
|
|
automatically, but first we need to figure out how it should choose and
|
|
how to enable you to guide the choice. No solution is evident.
|
|
|
|
Defining a global register variable in a certain register reserves that
|
|
register entirely for this use, at least within the current compilation.
|
|
The register will not be allocated for any other purpose in the functions
|
|
in the current compilation. The register will not be saved and restored by
|
|
these functions. Stores into this register are never deleted even if they
|
|
would appear to be dead, but references may be deleted or moved or
|
|
simplified.
|
|
|
|
It is not safe to access the global register variables from signal
|
|
handlers, or from more than one thread of control, because the system
|
|
library routines may temporarily use the register for other things (unless
|
|
you recompile them specially for the task at hand).
|
|
|
|
@cindex @code{qsort}, and global register variables
|
|
It is not safe for one function that uses a global register variable to
|
|
call another such function @code{foo} by way of a third function
|
|
@code{lose} that was compiled without knowledge of this variable (i.e. in a
|
|
different source file in which the variable wasn't declared). This is
|
|
because @code{lose} might save the register and put some other value there.
|
|
For example, you can't expect a global register variable to be available in
|
|
the comparison-function that you pass to @code{qsort}, since @code{qsort}
|
|
might have put something else in that register. (If you are prepared to
|
|
recompile @code{qsort} with the same global register variable, you can
|
|
solve this problem.)
|
|
|
|
If you want to recompile @code{qsort} or other source files which do not
|
|
actually use your global register variable, so that they will not use that
|
|
register for any other purpose, then it suffices to specify the compiler
|
|
option @samp{-ffixed-@var{reg}}. You need not actually add a global
|
|
register declaration to their source code.
|
|
|
|
A function which can alter the value of a global register variable cannot
|
|
safely be called from a function compiled without this variable, because it
|
|
could clobber the value the caller expects to find there on return.
|
|
Therefore, the function which is the entry point into the part of the
|
|
program that uses the global register variable must explicitly save and
|
|
restore the value which belongs to its caller.
|
|
|
|
@cindex register variable after @code{longjmp}
|
|
@cindex global register after @code{longjmp}
|
|
@cindex value after @code{longjmp}
|
|
@findex longjmp
|
|
@findex setjmp
|
|
On most machines, @code{longjmp} will restore to each global register
|
|
variable the value it had at the time of the @code{setjmp}. On some
|
|
machines, however, @code{longjmp} will not change the value of global
|
|
register variables. To be portable, the function that called @code{setjmp}
|
|
should make other arrangements to save the values of the global register
|
|
variables, and to restore them in a @code{longjmp}. This way, the same
|
|
thing will happen regardless of what @code{longjmp} does.
|
|
|
|
All global register variable declarations must precede all function
|
|
definitions. If such a declaration could appear after function
|
|
definitions, the declaration would be too late to prevent the register from
|
|
being used for other purposes in the preceding functions.
|
|
|
|
Global register variables may not have initial values, because an
|
|
executable file has no means to supply initial contents for a register.
|
|
|
|
On the Sparc, there are reports that g3 @dots{} g7 are suitable
|
|
registers, but certain library functions, such as @code{getwd}, as well
|
|
as the subroutines for division and remainder, modify g3 and g4. g1 and
|
|
g2 are local temporaries.
|
|
|
|
On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7.
|
|
Of course, it will not do to use more than a few of those.
|
|
|
|
@node Local Reg Vars
|
|
@subsection Specifying Registers for Local Variables
|
|
@cindex local variables, specifying registers
|
|
@cindex specifying registers for local variables
|
|
@cindex registers for local variables
|
|
|
|
You can define a local register variable with a specified register
|
|
like this:
|
|
|
|
@example
|
|
register int *foo asm ("a5");
|
|
@end example
|
|
|
|
@noindent
|
|
Here @code{a5} is the name of the register which should be used. Note
|
|
that this is the same syntax used for defining global register
|
|
variables, but for a local variable it would appear within a function.
|
|
|
|
Naturally the register name is cpu-dependent, but this is not a
|
|
problem, since specific registers are most often useful with explicit
|
|
assembler instructions (@pxref{Extended Asm}). Both of these things
|
|
generally require that you conditionalize your program according to
|
|
cpu type.
|
|
|
|
In addition, operating systems on one type of cpu may differ in how they
|
|
name the registers; then you would need additional conditionals. For
|
|
example, some 68000 operating systems call this register @code{%a5}.
|
|
|
|
Defining such a register variable does not reserve the register; it
|
|
remains available for other uses in places where flow control determines
|
|
the variable's value is not live. However, these registers are made
|
|
unavailable for use in the reload pass; excessive use of this feature
|
|
leaves the compiler too few available registers to compile certain
|
|
functions.
|
|
|
|
This option does not guarantee that GNU CC will generate code that has
|
|
this variable in the register you specify at all times. You may not
|
|
code an explicit reference to this register in an @code{asm} statement
|
|
and assume it will always refer to this variable.
|
|
|
|
@node Alternate Keywords
|
|
@section Alternate Keywords
|
|
@cindex alternate keywords
|
|
@cindex keywords, alternate
|
|
|
|
The option @samp{-traditional} disables certain keywords; @samp{-ansi}
|
|
disables certain others. This causes trouble when you want to use GNU C
|
|
extensions, or ANSI C features, in a general-purpose header file that
|
|
should be usable by all programs, including ANSI C programs and traditional
|
|
ones. The keywords @code{asm}, @code{typeof} and @code{inline} cannot be
|
|
used since they won't work in a program compiled with @samp{-ansi}, while
|
|
the keywords @code{const}, @code{volatile}, @code{signed}, @code{typeof}
|
|
and @code{inline} won't work in a program compiled with
|
|
@samp{-traditional}.@refill
|
|
|
|
The way to solve these problems is to put @samp{__} at the beginning and
|
|
end of each problematical keyword. For example, use @code{__asm__}
|
|
instead of @code{asm}, @code{__const__} instead of @code{const}, and
|
|
@code{__inline__} instead of @code{inline}.
|
|
|
|
Other C compilers won't accept these alternative keywords; if you want to
|
|
compile with another compiler, you can define the alternate keywords as
|
|
macros to replace them with the customary keywords. It looks like this:
|
|
|
|
@example
|
|
#ifndef __GNUC__
|
|
#define __asm__ asm
|
|
#endif
|
|
@end example
|
|
|
|
@samp{-pedantic} causes warnings for many GNU C extensions. You can
|
|
prevent such warnings within one expression by writing
|
|
@code{__extension__} before the expression. @code{__extension__} has no
|
|
effect aside from this.
|
|
|
|
@node Incomplete Enums
|
|
@section Incomplete @code{enum} Types
|
|
|
|
You can define an @code{enum} tag without specifying its possible values.
|
|
This results in an incomplete type, much like what you get if you write
|
|
@code{struct foo} without describing the elements. A later declaration
|
|
which does specify the possible values completes the type.
|
|
|
|
You can't allocate variables or storage using the type while it is
|
|
incomplete. However, you can work with pointers to that type.
|
|
|
|
This extension may not be very useful, but it makes the handling of
|
|
@code{enum} more consistent with the way @code{struct} and @code{union}
|
|
are handled.
|
|
|
|
This extension is not supported by GNU C++.
|
|
|
|
@node Function Names
|
|
@section Function Names as Strings
|
|
|
|
GNU CC predefines two string variables to be the name of the current function.
|
|
The variable @code{__FUNCTION__} is the name of the function as it appears
|
|
in the source. The variable @code{__PRETTY_FUNCTION__} is the name of
|
|
the function pretty printed in a language specific fashion.
|
|
|
|
These names are always the same in a C function, but in a C++ function
|
|
they may be different. For example, this program:
|
|
|
|
@smallexample
|
|
extern "C" @{
|
|
extern int printf (char *, ...);
|
|
@}
|
|
|
|
class a @{
|
|
public:
|
|
sub (int i)
|
|
@{
|
|
printf ("__FUNCTION__ = %s\n", __FUNCTION__);
|
|
printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
|
|
@}
|
|
@};
|
|
|
|
int
|
|
main (void)
|
|
@{
|
|
a ax;
|
|
ax.sub (0);
|
|
return 0;
|
|
@}
|
|
@end smallexample
|
|
|
|
@noindent
|
|
gives this output:
|
|
|
|
@smallexample
|
|
__FUNCTION__ = sub
|
|
__PRETTY_FUNCTION__ = int a::sub (int)
|
|
@end smallexample
|
|
|
|
These names are not macros: they are predefined string variables.
|
|
For example, @samp{#ifdef __FUNCTION__} does not have any special
|
|
meaning inside a function, since the preprocessor does not do anything
|
|
special with the identifier @code{__FUNCTION__}.
|
|
|
|
@node Return Address
|
|
@section Getting the Return or Frame Address of a Function
|
|
|
|
These functions may be used to get information about the callers of a
|
|
function.
|
|
|
|
@table @code
|
|
@item __builtin_return_address (@var{level})
|
|
This function returns the return address of the current function, or of
|
|
one of its callers. The @var{level} argument is number of frames to
|
|
scan up the call stack. A value of @code{0} yields the return address
|
|
of the current function, a value of @code{1} yields the return address
|
|
of the caller of the current function, and so forth.
|
|
|
|
The @var{level} argument must be a constant integer.
|
|
|
|
On some machines it may be impossible to determine the return address of
|
|
any function other than the current one; in such cases, or when the top
|
|
of the stack has been reached, this function will return @code{0}.
|
|
|
|
This function should only be used with a non-zero argument for debugging
|
|
purposes.
|
|
|
|
@item __builtin_frame_address (@var{level})
|
|
This function is similar to @code{__builtin_return_address}, but it
|
|
returns the address of the function frame rather than the return address
|
|
of the function. Calling @code{__builtin_frame_address} with a value of
|
|
@code{0} yields the frame address of the current function, a value of
|
|
@code{1} yields the frame address of the caller of the current function,
|
|
and so forth.
|
|
|
|
The frame is the area on the stack which holds local variables and saved
|
|
registers. The frame address is normally the address of the first word
|
|
pushed on to the stack by the function. However, the exact definition
|
|
depends upon the processor and the calling convention. If the processor
|
|
has a dedicated frame pointer register, and the function has a frame,
|
|
then @code{__builtin_frame_address} will return the value of the frame
|
|
pointer register.
|
|
|
|
The caveats that apply to @code{__builtin_return_address} apply to this
|
|
function as well.
|
|
@end table
|
|
|
|
@node C++ Extensions
|
|
@chapter Extensions to the C++ Language
|
|
@cindex extensions, C++ language
|
|
@cindex C++ language extensions
|
|
|
|
The GNU compiler provides these extensions to the C++ language (and you
|
|
can also use most of the C language extensions in your C++ programs). If you
|
|
want to write code that checks whether these features are available, you can
|
|
test for the GNU compiler the same way as for C programs: check for a
|
|
predefined macro @code{__GNUC__}. You can also use @code{__GNUG__} to
|
|
test specifically for GNU C++ (@pxref{Standard Predefined,,Standard
|
|
Predefined Macros,cpp.info,The C Preprocessor}).
|
|
|
|
@menu
|
|
* Naming Results:: Giving a name to C++ function return values.
|
|
* Min and Max:: C++ Minimum and maximum operators.
|
|
* Destructors and Goto:: Goto is safe to use in C++ even when destructors
|
|
are needed.
|
|
* C++ Interface:: You can use a single C++ header file for both
|
|
declarations and definitions.
|
|
* Template Instantiation:: Methods for ensuring that exactly one copy of
|
|
each needed template instantiation is emitted.
|
|
* C++ Signatures:: You can specify abstract types to get subtype
|
|
polymorphism independent from inheritance.
|
|
@end menu
|
|
|
|
@node Naming Results
|
|
@section Named Return Values in C++
|
|
|
|
@cindex @code{return}, in C++ function header
|
|
@cindex return value, named, in C++
|
|
@cindex named return value in C++
|
|
@cindex C++ named return value
|
|
GNU C++ extends the function-definition syntax to allow you to specify a
|
|
name for the result of a function outside the body of the definition, in
|
|
C++ programs:
|
|
|
|
@example
|
|
@group
|
|
@var{type}
|
|
@var{functionname} (@var{args}) return @var{resultname};
|
|
@{
|
|
@dots{}
|
|
@var{body}
|
|
@dots{}
|
|
@}
|
|
@end group
|
|
@end example
|
|
|
|
You can use this feature to avoid an extra constructor call when
|
|
a function result has a class type. For example, consider a function
|
|
@code{m}, declared as @w{@samp{X v = m ();}}, whose result is of class
|
|
@code{X}:
|
|
|
|
@example
|
|
X
|
|
m ()
|
|
@{
|
|
X b;
|
|
b.a = 23;
|
|
return b;
|
|
@}
|
|
@end example
|
|
|
|
@cindex implicit argument: return value
|
|
Although @code{m} appears to have no arguments, in fact it has one implicit
|
|
argument: the address of the return value. At invocation, the address
|
|
of enough space to hold @code{v} is sent in as the implicit argument.
|
|
Then @code{b} is constructed and its @code{a} field is set to the value
|
|
23. Finally, a copy constructor (a constructor of the form @samp{X(X&)})
|
|
is applied to @code{b}, with the (implicit) return value location as the
|
|
target, so that @code{v} is now bound to the return value.
|
|
|
|
But this is wasteful. The local @code{b} is declared just to hold
|
|
something that will be copied right out. While a compiler that
|
|
combined an ``elision'' algorithm with interprocedural data flow
|
|
analysis could conceivably eliminate all of this, it is much more
|
|
practical to allow you to assist the compiler in generating
|
|
efficient code by manipulating the return value explicitly,
|
|
thus avoiding the local variable and copy constructor altogether.
|
|
|
|
Using the extended GNU C++ function-definition syntax, you can avoid the
|
|
temporary allocation and copying by naming @code{r} as your return value
|
|
at the outset, and assigning to its @code{a} field directly:
|
|
|
|
@example
|
|
X
|
|
m () return r;
|
|
@{
|
|
r.a = 23;
|
|
@}
|
|
@end example
|
|
|
|
@noindent
|
|
The declaration of @code{r} is a standard, proper declaration, whose effects
|
|
are executed @strong{before} any of the body of @code{m}.
|
|
|
|
Functions of this type impose no additional restrictions; in particular,
|
|
you can execute @code{return} statements, or return implicitly by
|
|
reaching the end of the function body (``falling off the edge'').
|
|
Cases like
|
|
|
|
@example
|
|
X
|
|
m () return r (23);
|
|
@{
|
|
return;
|
|
@}
|
|
@end example
|
|
|
|
@noindent
|
|
(or even @w{@samp{X m () return r (23); @{ @}}}) are unambiguous, since
|
|
the return value @code{r} has been initialized in either case. The
|
|
following code may be hard to read, but also works predictably:
|
|
|
|
@example
|
|
X
|
|
m () return r;
|
|
@{
|
|
X b;
|
|
return b;
|
|
@}
|
|
@end example
|
|
|
|
The return value slot denoted by @code{r} is initialized at the outset,
|
|
but the statement @samp{return b;} overrides this value. The compiler
|
|
deals with this by destroying @code{r} (calling the destructor if there
|
|
is one, or doing nothing if there is not), and then reinitializing
|
|
@code{r} with @code{b}.
|
|
|
|
This extension is provided primarily to help people who use overloaded
|
|
operators, where there is a great need to control not just the
|
|
arguments, but the return values of functions. For classes where the
|
|
copy constructor incurs a heavy performance penalty (especially in the
|
|
common case where there is a quick default constructor), this is a major
|
|
savings. The disadvantage of this extension is that you do not control
|
|
when the default constructor for the return value is called: it is
|
|
always called at the beginning.
|
|
|
|
@node Min and Max
|
|
@section Minimum and Maximum Operators in C++
|
|
|
|
It is very convenient to have operators which return the ``minimum'' or the
|
|
``maximum'' of two arguments. In GNU C++ (but not in GNU C),
|
|
|
|
@table @code
|
|
@item @var{a} <? @var{b}
|
|
@findex <?
|
|
@cindex minimum operator
|
|
is the @dfn{minimum}, returning the smaller of the numeric values
|
|
@var{a} and @var{b};
|
|
|
|
@item @var{a} >? @var{b}
|
|
@findex >?
|
|
@cindex maximum operator
|
|
is the @dfn{maximum}, returning the larger of the numeric values @var{a}
|
|
and @var{b}.
|
|
@end table
|
|
|
|
These operations are not primitive in ordinary C++, since you can
|
|
use a macro to return the minimum of two things in C++, as in the
|
|
following example.
|
|
|
|
@example
|
|
#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
|
|
@end example
|
|
|
|
@noindent
|
|
You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to
|
|
the minimum value of variables @var{i} and @var{j}.
|
|
|
|
However, side effects in @code{X} or @code{Y} may cause unintended
|
|
behavior. For example, @code{MIN (i++, j++)} will fail, incrementing
|
|
the smaller counter twice. A GNU C extension allows you to write safe
|
|
macros that avoid this kind of problem (@pxref{Naming Types,,Naming an
|
|
Expression's Type}). However, writing @code{MIN} and @code{MAX} as
|
|
macros also forces you to use function-call notation for a
|
|
fundamental arithmetic operation. Using GNU C++ extensions, you can
|
|
write @w{@samp{int min = i <? j;}} instead.
|
|
|
|
Since @code{<?} and @code{>?} are built into the compiler, they properly
|
|
handle expressions with side-effects; @w{@samp{int min = i++ <? j++;}}
|
|
works correctly.
|
|
|
|
@node Destructors and Goto
|
|
@section @code{goto} and Destructors in GNU C++
|
|
|
|
@cindex @code{goto} in C++
|
|
@cindex destructors vs @code{goto}
|
|
In C++ programs, you can safely use the @code{goto} statement. When you
|
|
use it to exit a block which contains aggregates requiring destructors,
|
|
the destructors will run before the @code{goto} transfers control.
|
|
|
|
@cindex constructors vs @code{goto}
|
|
The compiler still forbids using @code{goto} to @emph{enter} a scope
|
|
that requires constructors.
|
|
|
|
@node C++ Interface
|
|
@section Declarations and Definitions in One Header
|
|
|
|
@cindex interface and implementation headers, C++
|
|
@cindex C++ interface and implementation headers
|
|
C++ object definitions can be quite complex. In principle, your source
|
|
code will need two kinds of things for each object that you use across
|
|
more than one source file. First, you need an @dfn{interface}
|
|
specification, describing its structure with type declarations and
|
|
function prototypes. Second, you need the @dfn{implementation} itself.
|
|
It can be tedious to maintain a separate interface description in a
|
|
header file, in parallel to the actual implementation. It is also
|
|
dangerous, since separate interface and implementation definitions may
|
|
not remain parallel.
|
|
|
|
@cindex pragmas, interface and implementation
|
|
With GNU C++, you can use a single header file for both purposes.
|
|
|
|
@quotation
|
|
@emph{Warning:} The mechanism to specify this is in transition. For the
|
|
nonce, you must use one of two @code{#pragma} commands; in a future
|
|
release of GNU C++, an alternative mechanism will make these
|
|
@code{#pragma} commands unnecessary.
|
|
@end quotation
|
|
|
|
The header file contains the full definitions, but is marked with
|
|
@samp{#pragma interface} in the source code. This allows the compiler
|
|
to use the header file only as an interface specification when ordinary
|
|
source files incorporate it with @code{#include}. In the single source
|
|
file where the full implementation belongs, you can use either a naming
|
|
convention or @samp{#pragma implementation} to indicate this alternate
|
|
use of the header file.
|
|
|
|
@table @code
|
|
@item #pragma interface
|
|
@itemx #pragma interface "@var{subdir}/@var{objects}.h"
|
|
@kindex #pragma interface
|
|
Use this directive in @emph{header files} that define object classes, to save
|
|
space in most of the object files that use those classes. Normally,
|
|
local copies of certain information (backup copies of inline member
|
|
functions, debugging information, and the internal tables that implement
|
|
virtual functions) must be kept in each object file that includes class
|
|
definitions. You can use this pragma to avoid such duplication. When a
|
|
header file containing @samp{#pragma interface} is included in a
|
|
compilation, this auxiliary information will not be generated (unless
|
|
the main input source file itself uses @samp{#pragma implementation}).
|
|
Instead, the object files will contain references to be resolved at link
|
|
time.
|
|
|
|
The second form of this directive is useful for the case where you have
|
|
multiple headers with the same name in different directories. If you
|
|
use this form, you must specify the same string to @samp{#pragma
|
|
implementation}.
|
|
|
|
@item #pragma implementation
|
|
@itemx #pragma implementation "@var{objects}.h"
|
|
@kindex #pragma implementation
|
|
Use this pragma in a @emph{main input file}, when you want full output from
|
|
included header files to be generated (and made globally visible). The
|
|
included header file, in turn, should use @samp{#pragma interface}.
|
|
Backup copies of inline member functions, debugging information, and the
|
|
internal tables used to implement virtual functions are all generated in
|
|
implementation files.
|
|
|
|
@cindex implied @code{#pragma implementation}
|
|
@cindex @code{#pragma implementation}, implied
|
|
@cindex naming convention, implementation headers
|
|
If you use @samp{#pragma implementation} with no argument, it applies to
|
|
an include file with the same basename@footnote{A file's @dfn{basename}
|
|
was the name stripped of all leading path information and of trailing
|
|
suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source
|
|
file. For example, in @file{allclass.cc}, giving just
|
|
@samp{#pragma implementation}
|
|
by itself is equivalent to @samp{#pragma implementation "allclass.h"}.
|
|
|
|
In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as
|
|
an implementation file whenever you would include it from
|
|
@file{allclass.cc} even if you never specified @samp{#pragma
|
|
implementation}. This was deemed to be more trouble than it was worth,
|
|
however, and disabled.
|
|
|
|
If you use an explicit @samp{#pragma implementation}, it must appear in
|
|
your source file @emph{before} you include the affected header files.
|
|
|
|
Use the string argument if you want a single implementation file to
|
|
include code from multiple header files. (You must also use
|
|
@samp{#include} to include the header file; @samp{#pragma
|
|
implementation} only specifies how to use the file---it doesn't actually
|
|
include it.)
|
|
|
|
There is no way to split up the contents of a single header file into
|
|
multiple implementation files.
|
|
@end table
|
|
|
|
@cindex inlining and C++ pragmas
|
|
@cindex C++ pragmas, effect on inlining
|
|
@cindex pragmas in C++, effect on inlining
|
|
@samp{#pragma implementation} and @samp{#pragma interface} also have an
|
|
effect on function inlining.
|
|
|
|
If you define a class in a header file marked with @samp{#pragma
|
|
interface}, the effect on a function defined in that class is similar to
|
|
an explicit @code{extern} declaration---the compiler emits no code at
|
|
all to define an independent version of the function. Its definition
|
|
is used only for inlining with its callers.
|
|
|
|
Conversely, when you include the same header file in a main source file
|
|
that declares it as @samp{#pragma implementation}, the compiler emits
|
|
code for the function itself; this defines a version of the function
|
|
that can be found via pointers (or by callers compiled without
|
|
inlining). If all calls to the function can be inlined, you can avoid
|
|
emitting the function by compiling with @samp{-fno-implement-inlines}.
|
|
If any calls were not inlined, you will get linker errors.
|
|
|
|
@node Template Instantiation
|
|
@section Where's the Template?
|
|
|
|
@cindex template instantiation
|
|
|
|
C++ templates are the first language feature to require more
|
|
intelligence from the environment than one usually finds on a UNIX
|
|
system. Somehow the compiler and linker have to make sure that each
|
|
template instance occurs exactly once in the executable if it is needed,
|
|
and not at all otherwise. There are two basic approaches to this
|
|
problem, which I will refer to as the Borland model and the Cfront model.
|
|
|
|
@table @asis
|
|
@item Borland model
|
|
Borland C++ solved the template instantiation problem by adding the code
|
|
equivalent of common blocks to their linker; the compiler emits template
|
|
instances in each translation unit that uses them, and the linker
|
|
collapses them together. The advantage of this model is that the linker
|
|
only has to consider the object files themselves; there is no external
|
|
complexity to worry about. This disadvantage is that compilation time
|
|
is increased because the template code is being compiled repeatedly.
|
|
Code written for this model tends to include definitions of all
|
|
templates in the header file, since they must be seen to be
|
|
instantiated.
|
|
|
|
@item Cfront model
|
|
The AT&T C++ translator, Cfront, solved the template instantiation
|
|
problem by creating the notion of a template repository, an
|
|
automatically maintained place where template instances are stored. A
|
|
more modern version of the repository works as follows: As individual
|
|
object files are built, the compiler places any template definitions and
|
|
instantiations encountered in the repository. At link time, the link
|
|
wrapper adds in the objects in the repository and compiles any needed
|
|
instances that were not previously emitted. The advantages of this
|
|
model are more optimal compilation speed and the ability to use the
|
|
system linker; to implement the Borland model a compiler vendor also
|
|
needs to replace the linker. The disadvantages are vastly increased
|
|
complexity, and thus potential for error; for some code this can be
|
|
just as transparent, but in practice it can been very difficult to build
|
|
multiple programs in one directory and one program in multiple
|
|
directories. Code written for this model tends to separate definitions
|
|
of non-inline member templates into a separate file, which should be
|
|
compiled separately.
|
|
@end table
|
|
|
|
When used with GNU ld version 2.8 or later on an ELF system such as
|
|
Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the
|
|
Borland model. On other systems, g++ implements neither automatic
|
|
model.
|
|
|
|
A future version of g++ will support a hybrid model whereby the compiler
|
|
will emit any instantiations for which the template definition is
|
|
included in the compile, and store template definitions and
|
|
instantiation context information into the object file for the rest.
|
|
The link wrapper will extract that information as necessary and invoke
|
|
the compiler to produce the remaining instantiations. The linker will
|
|
then combine duplicate instantiations.
|
|
|
|
In the mean time, you have the following options for dealing with
|
|
template instantiations:
|
|
|
|
@enumerate
|
|
@item
|
|
Compile your template-using code with @samp{-frepo}. The compiler will
|
|
generate files with the extension @samp{.rpo} listing all of the
|
|
template instantiations used in the corresponding object files which
|
|
could be instantiated there; the link wrapper, @samp{collect2}, will
|
|
then update the @samp{.rpo} files to tell the compiler where to place
|
|
those instantiations and rebuild any affected object files. The
|
|
link-time overhead is negligible after the first pass, as the compiler
|
|
will continue to place the instantiations in the same files.
|
|
|
|
This is your best option for application code written for the Borland
|
|
model, as it will just work. Code written for the Cfront model will
|
|
need to be modified so that the template definitions are available at
|
|
one or more points of instantiation; usually this is as simple as adding
|
|
@code{#include <tmethods.cc>} to the end of each template header.
|
|
|
|
For library code, if you want the library to provide all of the template
|
|
instantiations it needs, just try to link all of its object files
|
|
together; the link will fail, but cause the instantiations to be
|
|
generated as a side effect. Be warned, however, that this may cause
|
|
conflicts if multiple libraries try to provide the same instantiations.
|
|
For greater control, use explicit instantiation as described in the next
|
|
option.
|
|
|
|
@item
|
|
Compile your code with @samp{-fno-implicit-templates} to disable the
|
|
implicit generation of template instances, and explicitly instantiate
|
|
all the ones you use. This approach requires more knowledge of exactly
|
|
which instances you need than do the others, but it's less
|
|
mysterious and allows greater control. You can scatter the explicit
|
|
instantiations throughout your program, perhaps putting them in the
|
|
translation units where the instances are used or the translation units
|
|
that define the templates themselves; you can put all of the explicit
|
|
instantiations you need into one big file; or you can create small files
|
|
like
|
|
|
|
@example
|
|
#include "Foo.h"
|
|
#include "Foo.cc"
|
|
|
|
template class Foo<int>;
|
|
template ostream& operator <<
|
|
(ostream&, const Foo<int>&);
|
|
@end example
|
|
|
|
for each of the instances you need, and create a template instantiation
|
|
library from those.
|
|
|
|
If you are using Cfront-model code, you can probably get away with not
|
|
using @samp{-fno-implicit-templates} when compiling files that don't
|
|
@samp{#include} the member template definitions.
|
|
|
|
If you use one big file to do the instantiations, you may want to
|
|
compile it without @samp{-fno-implicit-templates} so you get all of the
|
|
instances required by your explicit instantiations (but not by any
|
|
other files) without having to specify them as well.
|
|
|
|
g++ has extended the template instantiation syntax outlined in the
|
|
Working Paper to allow forward declaration of explicit instantiations,
|
|
explicit instantiation of members of template classes and instantiation
|
|
of the compiler support data for a template class (i.e. the vtable)
|
|
without instantiating any of its members:
|
|
|
|
@example
|
|
extern template int max (int, int);
|
|
template void Foo<int>::f ();
|
|
inline template class Foo<int>;
|
|
@end example
|
|
|
|
@item
|
|
Do nothing. Pretend g++ does implement automatic instantiation
|
|
management. Code written for the Borland model will work fine, but
|
|
each translation unit will contain instances of each of the templates it
|
|
uses. In a large program, this can lead to an unacceptable amount of code
|
|
duplication.
|
|
|
|
@item
|
|
Add @samp{#pragma interface} to all files containing template
|
|
definitions. For each of these files, add @samp{#pragma implementation
|
|
"@var{filename}"} to the top of some @samp{.C} file which
|
|
@samp{#include}s it. Then compile everything with
|
|
@samp{-fexternal-templates}. The templates will then only be expanded
|
|
in the translation unit which implements them (i.e. has a @samp{#pragma
|
|
implementation} line for the file where they live); all other files will
|
|
use external references. If you're lucky, everything should work
|
|
properly. If you get undefined symbol errors, you need to make sure
|
|
that each template instance which is used in the program is used in the
|
|
file which implements that template. If you don't have any use for a
|
|
particular instance in that file, you can just instantiate it
|
|
explicitly, using the syntax from the latest C++ working paper:
|
|
|
|
@example
|
|
template class A<int>;
|
|
template ostream& operator << (ostream&, const A<int>&);
|
|
@end example
|
|
|
|
This strategy will work with code written for either model. If you are
|
|
using code written for the Cfront model, the file containing a class
|
|
template and the file containing its member templates should be
|
|
implemented in the same translation unit.
|
|
|
|
A slight variation on this approach is to instead use the flag
|
|
@samp{-falt-external-templates}; this flag causes template
|
|
instances to be emitted in the translation unit that implements the
|
|
header where they are first instantiated, rather than the one which
|
|
implements the file where the templates are defined. This header must
|
|
be the same in all translation units, or things are likely to break.
|
|
|
|
@xref{C++ Interface,,Declarations and Definitions in One Header}, for
|
|
more discussion of these pragmas.
|
|
@end enumerate
|
|
|
|
@node C++ Signatures
|
|
@section Type Abstraction using Signatures
|
|
|
|
@findex signature
|
|
@cindex type abstraction, C++
|
|
@cindex C++ type abstraction
|
|
@cindex subtype polymorphism, C++
|
|
@cindex C++ subtype polymorphism
|
|
@cindex signatures, C++
|
|
@cindex C++ signatures
|
|
|
|
In GNU C++, you can use the keyword @code{signature} to define a
|
|
completely abstract class interface as a datatype. You can connect this
|
|
abstraction with actual classes using signature pointers. If you want
|
|
to use signatures, run the GNU compiler with the
|
|
@samp{-fhandle-signatures} command-line option. (With this option, the
|
|
compiler reserves a second keyword @code{sigof} as well, for a future
|
|
extension.)
|
|
|
|
Roughly, signatures are type abstractions or interfaces of classes.
|
|
Some other languages have similar facilities. C++ signatures are
|
|
related to ML's signatures, Haskell's type classes, definition modules
|
|
in Modula-2, interface modules in Modula-3, abstract types in Emerald,
|
|
type modules in Trellis/Owl, categories in Scratchpad II, and types in
|
|
POOL-I. For a more detailed discussion of signatures, see
|
|
@cite{Signatures: A Language Extension for Improving Type Abstraction and
|
|
Subtype Polymorphism in C++}
|
|
by @w{Gerald} Baumgartner and Vincent F. Russo (Tech report
|
|
CSD--TR--95--051, Dept. of Computer Sciences, Purdue University,
|
|
August 1995, a slightly improved version appeared in
|
|
@emph{Software---Practice & Experience}, @b{25}(8), pp. 863--889,
|
|
August 1995). You can get the tech report by anonymous FTP from
|
|
@code{ftp.cs.purdue.edu} in @file{pub/gb/Signature-design.ps.gz}.
|
|
|
|
Syntactically, a signature declaration is a collection of
|
|
member function declarations and nested type declarations.
|
|
For example, this signature declaration defines a new abstract type
|
|
@code{S} with member functions @samp{int foo ()} and @samp{int bar (int)}:
|
|
|
|
@example
|
|
signature S
|
|
@{
|
|
int foo ();
|
|
int bar (int);
|
|
@};
|
|
@end example
|
|
|
|
Since signature types do not include implementation definitions, you
|
|
cannot write an instance of a signature directly. Instead, you can
|
|
define a pointer to any class that contains the required interfaces as a
|
|
@dfn{signature pointer}. Such a class @dfn{implements} the signature
|
|
type.
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|
@c Eventually signature references should work too.
|
|
|
|
To use a class as an implementation of @code{S}, you must ensure that
|
|
the class has public member functions @samp{int foo ()} and @samp{int
|
|
bar (int)}. The class can have other member functions as well, public
|
|
or not; as long as it offers what's declared in the signature, it is
|
|
suitable as an implementation of that signature type.
|
|
|
|
For example, suppose that @code{C} is a class that meets the
|
|
requirements of signature @code{S} (@code{C} @dfn{conforms to}
|
|
@code{S}). Then
|
|
|
|
@example
|
|
C obj;
|
|
S * p = &obj;
|
|
@end example
|
|
|
|
@noindent
|
|
defines a signature pointer @code{p} and initializes it to point to an
|
|
object of type @code{C}.
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|
The member function call @w{@samp{int i = p->foo ();}}
|
|
executes @samp{obj.foo ()}.
|
|
|
|
@cindex @code{signature} in C++, advantages
|
|
Abstract virtual classes provide somewhat similar facilities in standard
|
|
C++. There are two main advantages to using signatures instead:
|
|
|
|
@enumerate
|
|
@item
|
|
Subtyping becomes independent from inheritance. A class or signature
|
|
type @code{T} is a subtype of a signature type @code{S} independent of
|
|
any inheritance hierarchy as long as all the member functions declared
|
|
in @code{S} are also found in @code{T}. So you can define a subtype
|
|
hierarchy that is completely independent from any inheritance
|
|
(implementation) hierarchy, instead of being forced to use types that
|
|
mirror the class inheritance hierarchy.
|
|
|
|
@item
|
|
Signatures allow you to work with existing class hierarchies as
|
|
implementations of a signature type. If those class hierarchies are
|
|
only available in compiled form, you're out of luck with abstract virtual
|
|
classes, since an abstract virtual class cannot be retrofitted on top of
|
|
existing class hierarchies. So you would be required to write interface
|
|
classes as subtypes of the abstract virtual class.
|
|
@end enumerate
|
|
|
|
@cindex default implementation, signature member function
|
|
@cindex signature member function default implementation
|
|
There is one more detail about signatures. A signature declaration can
|
|
contain member function @emph{definitions} as well as member function
|
|
declarations. A signature member function with a full definition is
|
|
called a @emph{default implementation}; classes need not contain that
|
|
particular interface in order to conform. For example, a
|
|
class @code{C} can conform to the signature
|
|
|
|
@example
|
|
signature T
|
|
@{
|
|
int f (int);
|
|
int f0 () @{ return f (0); @};
|
|
@};
|
|
@end example
|
|
|
|
@noindent
|
|
whether or not @code{C} implements the member function @samp{int f0 ()}.
|
|
If you define @code{C::f0}, that definition takes precedence;
|
|
otherwise, the default implementation @code{S::f0} applies.
|
|
|
|
@ignore
|
|
There will be more support for signatures in the future.
|
|
Add to this doc as the implementation grows.
|
|
In particular, the following features are planned but not yet
|
|
implemented:
|
|
@itemize @bullet
|
|
@item signature references,
|
|
@item signature inheritance,
|
|
@item the @code{sigof} construct for extracting the signature information
|
|
of a class,
|
|
@item views for renaming member functions when matching a class type
|
|
with a signature type,
|
|
@item specifying exceptions with signature member functions, and
|
|
@item signature templates.
|
|
@end itemize
|
|
This list is roughly in the order in which we intend to implement
|
|
them. Watch this space for updates.
|
|
@end ignore
|